Patent Grant
7283145
1. Field of the Invention
The present invention relates to an image heating apparatus adapted for use as a heat fixing apparatus in a copying machine or a printer, and a heater adapted for use in such image heating apparatus.
2. Description of the Related Art
In a heat fixing apparatus for a copying machine or a printer, there is commercialized an apparatus of a configuration having, as disclosed in Japanese Patent Application Laid-open No. S63-313182, a flexible sleeve, a ceramic heater in contact with an internal surface of the flexible sleeve, and a pressure roller constituting a nip portion with the ceramic heater through the flexible sleeve, in which a recording material bearing a toner image is conveyed by the nip portion to heat fixing the toner image onto the recording material. Such heat fixing apparatus (called film heating type), having a very low heat capacity, has advantages of a quick warning up to a fixable temperature thereby providing a short print waiting time, and a low electric power consumption in a stand-by state waiting for a print command.
The flexible sleeve is made of polyimide or stainless steel. Also the ceramic heater is formed by printing a heat-generating resistor principally constituted of silver or palladium on a plate-shaped ceramic substrate excellent in heat resistance, thermal conductivity and electrical insulation such as of alumina or aluminum nitride. A temperature of the heater is controlled by controlling a current supply to the heat-generating resistor, based on a temperature detected by a thermistor maintained in contact with the ceramic heater.
Such fixing apparatus, though being excellent in the quick-starting property because of its low heat capacity, is associated with drawbacks because of such low heat capacity. In case the longitudinal length of the recording material is relatively short in comparison with the longitudinal length of the heater, an amount of heat taken away from the heater is different significantly, in the nip portion, between a sheet passing area passed by the recording material and a sheet non-passing area not passed by the recording material, so that the temperature of the sheet non-passing area, where the heat is not taken away by the recording material, is gradually elevated as the sheets are passed one by one. Thus there tends to result a temperature elevation phenomenon in the sheet non-passing area, which becomes more marked in the film heating system of low heat capacity. Since an excessive temperature elevation phenomenon in the sheet non-passing area causes a thermal deterioration of the components of the fixing apparatus thereby leading to a reduction in the service life of the apparatus, there have been proposed a heater configuration and a control method for the fixing apparatus for solving such drawbacks.
Japanese Patent Application Laid-open No. 2000-162909 proposes a method of reducing the aforementioned temperature elevation in the sheet non-passing area, utilizing a heater 700 of a structure as shown in
A heater 700 shown in
A heater driving circuit 70 shown in
In the fixing apparatus equipped with the heater 700 of
Also Japanese Patent Application Laid-open No. 2000-250337 proposes a similar heater configuration, in which three heat-generating patterns are independently activated as shown in
Also Japanese Patent Application Laid-open No. H10-177319 proposes a fixing apparatus employing a heater capable of forming an arc-shaped heat generation distribution by a multi-step heat generation control according to various sheet sizes, thereby suppressing the temperature elevation in the sheet non-passing area within a certain range while securing the fixing property.
A heater 900 shown in
With the heater 900, a smooth slope can be obtained in the heat generation distribution in the longitudinal direction, by incorporating the heater 900 in a heater driving circuit 70 shown in
However, in such fixing apparatus of film heating type utilizing such ceramic heater, in so-called uncontrollable situation of the fixing apparatus caused for example by a failure of the triac therein, the heater may show an excessive temperature increase and the ceramic substrate may be cracked by a thermal stress applied to the heater before the safety element (temperature fuse or thermo switch) can function. Also depending on the manner of cracking of the ceramic substrate, a dielectric strength cannot be satisfied between a resistance circuit (AC) side (primary side) including the heat generating pattern and a temperature sensor circuit (DC) side (secondary side) for heater temperature detection and the secondary circuit may be destructed by a current leaking to the main body of the image forming apparatus equipped with the fixing apparatus.
A thermal stress σ applied to a cross section of the substrate is represented, in case the temperature distribution is symmetrical within the cross section of the substrate, by a linear thermal expansion coefficient ε and a Young's modulus E of the substrate and a temperature difference ΔT within the substrate, which is dependent on the thermal conductivity thereof, by a following equation:
σ=ε·E·ΔT
However, in case the temperature distribution is asymmetrical, it no longer is simply proportional to the temperature difference ΔT because a bending moment is applied to the substrate, and the tensile stress generally becomes larger at the bending side of the substrate. A breakage occurs when such tensile stress exceeds the bending strength (breaking strength) of the substrate.
For example, in case of a heater bearing a heat-generating pattern along the longitudinal direction on a surface of an alumina substrate having a length of 370 mm, a width of 10 mm and a thickness of 1 mm, a largest thermal stress is known to occur in a cross section in the direction of width (shorter side) of the substrate. Therefore, the breakage of the heater by the thermal stress can be considered to depend largely on the temperature distribution in the direction of width (shorter side) of the substrate.
In a heater with prior plural drives, namely in a heater in which plural heat generating patterns are independently driven by plural triacs, in case of a thermal uncontrollable of the heater by a failure in a triac, the temperature distribution increases asymmetry in the cross section in the direction of width of the substrate, and a margin to the heater breakage is limited because of a strong tensile stress functioning at the same time.
For example, in the heater 700 shown in
In the heater 800 shown in
Also in the heater 900 shown in
The present invention has been made in consideration of the aforementioned drawbacks, and an object thereof is provide an image heating apparatus having an excellent durability of a heater, and a heater to be employed in such apparatus.
Another object of the present invention is to provide an image heating apparatus of which a heat generation distribution in the shorter side direction of the heater is more symmetrical than in the prior technology, with respect the center in the shorter side direction of the substrate, and a heater to be employed in such apparatus.
Still another object of the present invention is to provide an image heating apparatus including:
a heater including a substrate and a plurality of heat generating resistors formed on said substrate along a longitudinal direction thereof; and
a plurality of switching elements connected electrically between a power source and said plurality of heat generating resistors;
wherein said plurality of heat generating resistors include at least two first heat generating resistors driven by a first switching element and at least one of a second heat generating resistor driven by a second switching element, and said second heat generating resistor is provided between said first heat generating resistors in a direction of a shorter side of said substrate.
Still another object of the present invention is to provide a heater including:
a substrate; and
a plurality of heat generating resistors formed on said substrate along a longitudinal direction thereof;
wherein said plurality of heat generating resistors include at least two first heat generating resistors driven by a first switching element of the image heating apparatus and at least one of a second heat generating resistor driven by a second switching element of the image heating apparatus, and said second heat generating resistor is provided between said first heat generating resistors in a direction of a shorter side of said substrate.
Still other objects of the present invention will become fully apparent from the following detailed description, which is to be taken in conjunction with the accompanying drawings.
In the following examples of the present invention will be explained with reference to the accompanying drawings.
(1) Example of Image Forming Apparatus
The image forming apparatus is provided with an electrophotographic photosensitive member of drum shape (hereinafter represented as photosensitive drum) as an image bearing member. The photosensitive drum 1 is rotatably supported in a main body M of the apparatus, and is rotated at a predetermined process speed in a direction R1 by drive means (not shown).
Around the photosensitive drum 1 and along a rotating direction thereof, there are provided in succession a charging roller (charging apparatus) 2, exposure means 3, a developing apparatus 4, a transfer roller (transfer apparatus) 5 and a cleaning apparatus 6.
In a lower part of the main body M of the apparatus, there is provided a sheet cassette 7 containing sheet-shaped recording material P such as paper as the recording material, and along a conveying path of the recording material P and in succession from the upstream side, there are provided a sheet feeding roller 15, conveying rollers 8, a top sensor 9, a conveying guide 10, a fixing apparatus 11 containing a heater of the invention, conveying rollers 12, sheet discharge rollers 13 and a sheet discharge tray 14.
In the following, functions of the image forming apparatus of the above-described configuration will be explained.
The photosensitive drum 1, rotated in the direction R1 by the drive means (not shown), is uniformly charged by the charging roller 2 at a predetermined polarity and at a predetermined potential.
The photosensitive drum 1 after charging is subjected, by exposure means 3 such as a laser optical system, to an image exposure L based on image information, whereby a charge in an exposed portion is eliminated and an electrostatic latent image is formed.
The electrostatic latent image is developed by the developing apparatus 4. The developing apparatus 4 is provided with a developing roller 4a, which is given a developing bias and deposits a toner onto the electrostatic latent image on the photosensitive drum 1 thereby developing it into a toner image (visible image).
The toner image is transferred by the transfer roller 5 onto the recording material P such as paper. The recording material P is contained in the sheet cassette 7, and is fed and conveyed by the feeding roller 15 and the conveying rollers 8, through the top sensor 9, to a transfer nip portion between the photosensitive drum 1 and the transfer roller 5. In this operation, the recording material P is detected at a front end thereof by the top sensor 9 and is thus synchronized with the toner image on the photosensitive drum 1. The transfer roller 5 is given a transfer bias, by which the toner image on the photosensitive drum 1 is transferred onto a predetermined position on the recording material P.
The recording material P, bearing thereon the transferred and unfixed toner image, is conveyed along the conveying guide 10 to the fixing apparatus 11, in which the unfixed toner image is fixed by heat and pressure onto the surface of the recording material P. The fixing apparatus 11 will be explained later in more details.
The recording material P after the toner image fixation is conveyed by the conveying rollers 12 and discharge rollers 13 and discharged onto the discharge tray 14 provided on an upper surface of the main body M of the apparatus.
On the other hand, the photosensitive drum 1 after the toner image transfer is subjected to a removal of a toner that has not been transferred onto the recording material P but remains on the surface (hereinafter represented as transfer residual toner), by a cleaning blade 6a of the cleaning apparatus 6 and is thus prepared for a next image formation.
Image formations can be executed by repeating the aforementioned process.
(2) Fixing Apparatus 11
The fixing apparatus 11 of the present example is a pressure roller driving type, in which a heater support member 20 supporting a heater 100 is pressed to a pressure roller 40, constituting a pressure member, under a predetermined pressure through a cylindrical heat-resistant film 30 serving as a flexible sleeve, thereby forming a fixing nip portion N between the pressure roller and the heater 100.
When the pressure roller 40 is rotated in a direction b by a rotation control unit 80, the heat-resistant film 30 rotates, by a friction with the pressure roller 40, in a direction a around the external periphery of the heater support member 20 supporting the heater 100. On the other hand, a power supply to the heater is controlled by a heater driving circuit 70 in such a manner that a temperature detected by a temperature detector 50 maintains a target temperature, whereby the heater is maintained at about the target temperature. In such state, the recording material P bearing the unfixed toner image T is conveyed in the fixing nip portion N in a direction c, whereby the heat of the heater 100 is given through the heat-resistant film to the recording material P and the unfixed toner image T is thermally fixed onto the recording material P. The recording material P after passing the fixing nip portion N is separated by a curvature from the heat-resistant film 30 and discharged. In the present example, the passing of the recording material P is executed on a reference position at the center of the longitudinal direction (perpendicular to the conveying direction c of the recording material P) of each member.
The heater 100 is prepared by forming, on an oblong heat-resistant substrate 104 such as of alumina, three heat-generating patterns (heat generating resistors) 101a (101a-1 and 101a-2) and 101b, and a surface protective layer 106 for covering these resistors. The heater 100 will be explained in more details in following (3).
The cylindrical heat-resistant film 30 is a thin film tube having a polyimide base layer of a thickness of about 30-100 μm, and a coating of PFA or PTFE is provided across a primer layer on the base layer for providing a releasing property to the toner. Also grease (not shown) is coated between the internal surface of the film 30 and the heater support member 20 in order to secure a sliding property of the film 30.
The pressure roller 30 is a rotary member constituted by forming, on a metal core, an elastic layer such as of silicone rubber and further forming a releasing layer of FEP or PFA of thickness of about 10-100 μm across a primer layer, thereby securing a releasing property to the toner.
The heater support member 20 is formed by a heat-resistant resin having a heat insulating property, a high heat resistance and a rigidity such as polyphenylene sulfide (PPS), polyamidimide (PAI), polyimide (PI), polyether ether ketone (PEEK) or a liquid crystal polymer, or a composite material of such resin and ceramics, metal or glass.
The rotation control unit 80 is provided with a motor 81 for rotating the pressure roller 40, and a control unit (CPU) 82 for controlling the rotation of the motor 81. The motor 81 can be, for example, a DC motor or a stepping motor.
(3) Heater 100
The heater 100 is provided, on a surface of an oblong substrate 104 of a ceramic material having a high heat resistance, a electrical insulating property and a low heat capacity such as alumina or aluminum nitride (alumina in the present example 1) for example of a length of 370 mm, a width of 10 mm and a thickness of 1 mm, heat generating patterns 101a (101a-1, 101a-2) and 101b such as of Ag/Pd, and current feeding electrodes 102 (102a, 102b) and a common electrode 103 as electrode patterns for power supply to the heat generating patterns 101. The two heat generating patterns 101a (101a-1, 101a-2) (first heat generating resistors) are driven by a first switching element to be explained later, and the heat generating pattern 101b (second heat generating resistor) is driven by a second switching element to be explained later. The heat generating patterns 101a (101a-1, 101a-2) are driven (on/off controlled) by the first switching element and always execute heat generation at the same time.
In the following there will be explained detailed configuration of the heat generating patterns 101a-1 and 101a-2.
The heat generating patterns 101a-1, 101a-2 (first heat generating resistors), capable of passing a current from a current supply electrode 102a provided at a longitudinal end of a surface of the substrate to the common electrode 103, are provided at an end side and another end side in the direction of width (shorter side) of the substrate as shown in
The heat generating pattern 101b (second heat generating resistor), capable of passing a current from a current supply electrode 102b provided at a longitudinal end of a surface of the substrate to the common electrode 103, is provided, in the direction of width of the substrate, between the heat generating patterns 101a-1, 101a-2 (inner position than the first conductive path on the substrate) and constitutes a second conductive path along the longitudinal direction of the substrate 104. Also the heat generating pattern 101b is formed in substantially symmetrical areas with respect to the approximate shorter side center CL of the substrate. The heat generating pattern 101b is made narrower in the pattern width in the shorter side direction in plural steps from approximate center to both ends in the longitudinal direction to gradually increase the resistance per unit length in the longitudinal direction, thereby providing, when a current is passed, a concave heat generating distribution (hereinafter also called “concave type heat generation pattern”) having a bottom of heat generation at the approximate center. In the heat generating pattern 101b of the present example, the pattern width thereof is so regulated that a resistance per unit length in the longitudinal direction of the substrate in the vicinity of a line β-β at about the longitudinal center in
Also the heat generating patterns 101a and 101b are set at a resistance of Ra=20 Ω(Ra1=Ra2=10 Ω because of serial connection) and Rb=20 Ω, so that each heat generating pattern generates a power of 720 W under an application of 120 V. With such resistance setting, each heat generating pattern can be prepared with a same composition by selecting the pattern widths, on a line α-α, for example Wa1=Wa2=1.6 mm, Wb=0.8 mm and a pattern gap of 0.5 mm.
Also as shown in
Also a safety element 60 (temperature fuse or thermo switch) for preventing the excessive temperature elevation of the heater 100 is connected serially in the current supply line and is positioned in contact with the heater 100 or close thereto. In case of a thermal uncontrollable state of the heater 100 for example by a failure of the triac 72a or 72b, the safety element is activated in response to the heat of the heater 100 thereby terminating the current supply to the heater 100. The fixing apparatus of the present example employs a thermo switch CH-16 (manufactured by Wako Electronic Co., rated operation temperature: 250° C.) as the safety element 60. This thermo switch 60 is identified, in a preliminary testing, to function within a time of 10±1 seconds in case a uncontrollable state is caused by a failure of a triac (namely disabled temperature management by the CPU 71) and a power of 980 W (application of a voltage of 140 V to the resistor of 20 Ω), for example in case a power is continuously supplied without the temperature control to the heater from a state of normal temperature (24° C.).
The present example employs an alumina substrate 104 of a linear expansion coefficient ε=7.2×10−6/° C., a Young's modulus E=340 GPa and a bending strength of 400 MPa. Each thermal stress distribution shows a state after 3 seconds from the start of a thermal uncontrollable state caused by a failure of a triac in the course of current supply (application of a voltage of 140 V) to the heat generating resistors, and, in each chart, an upper part shows a compression stress and a lower area shows a tensile stress. As explained in the foregoing, a magnitude of the tensile stress is related with the breakage and a larger absolute value of the tensile stress results in a smaller margin to the breakage and a shorter time to the breakage.
At first, in case of a thermal uncontrollable of the convex type heat generating patterns 101a (first heat generating resistors) by a failure of the triac 72a, the absolute tensile stress became maximum at both ends of the α-α cross section in
Also in case of a thermal uncontrollable of the concave type heat generating pattern 101b by a failure of the triac 72b, the absolute tensile stress became maximum at both ends of the β-β cross section in
Now a heater 900 shown in
The heat generating pattern 901a is a single heat generating resistor capable of passing a current from the current supplying electrode 902a to the common electrode 903, and is widened in the pattern width in plural steps from approximate center to both ends in the longitudinal direction to gradually reduce the resistance per unit length in the longitudinal direction, thereby constituting a convex type heat generation pattern. In
The heat generating pattern 901b is a single heat generating resistor capable of passing a current from the current supplying electrode 902b to the common electrode 903, and is made narrower in the pattern width in plural steps from approximate center to both ends in the longitudinal direction to gradually increase the resistance per unit length in the longitudinal direction, thereby constituting a concave type heat generation pattern. In
The heat generating patterns 901a and 901b are set at a resistance of Ra=20 Ω and Rb=20 Ω, so that each heat generating pattern generates a power of 720 W under an application of 120 V. With such resistor setting, each heat generating pattern can be prepared with a same composition by selecting the pattern widths, on a line α-α in
Also as shown in
At first, in case of a thermal uncontrollable of the convex type heat generating patterns 901a by a failure of the triac 72a, the absolute tensile stress became maximum at both ends A1 of the α-α cross section in
Also in case of a thermal uncontrollable of the concave type heat generating pattern 101b by a failure of the triac 72b, the absolute tensile stress became maximum at both ends A2 of the β-β cross section in
As explained in the foregoing, the present example can significantly relax the thermal stress in a thermal uncontrollable state of the heat generating pattern in comparison with the comparative example, thereby securing a margin to the heat breakage. This is principally based on a level of symmetry of positioning of the heat generating patterns with respect to the approximate shorter side center CL of the substrate, and, in contrast to the prior plural heat generating patterns which are provided asymmetrically, the two heat generating patterns on a same conductive path are positioned at an edge side and at the other edge side in the direction of width of the substrate while a heat generating pattern on the other conductive path is positioned therebetween as described in the present example, whereby a symmetry of heat generation is secured with respect to the approximate shorter side center CL of the substrate when either pattern is energized. In this manner it is rendered possible to improve the durability and the reliability of the heater, and to improve the quality and the reliability of the fixing apparatus.
Stated differently, as the image heating apparatus includes “a substrate and plural heat generating resistors formed along a longitudinal direction of the substrate”, and plural switching elements connected between a power source and the plural heat generating elements; wherein the plural heat generating resistors include at least two first heat generating resistors driven by a first switching element, and at least one of a second heat generating resistor driven by a second switching element, and the second heat generating resistor is provided, in a shorter side direction of the substrate, between the at least two first heat generating resistors, it is rendered possible to improve the durability of the heater and to suppress a breakage of the heater before the function of the safety element.
It is also possible to reduce a temperature elevation in a sheet non-passing area and to secure the fixing property at the same time, in case the first heat generating resistors driven by the first switching element and the second heat generating resistor have different heat generating distributions.
The example 1 has explained a case of positioning the heat generating patterns of a convex heat generating distribution on both edge sides in the direction of width of the substrate and the heat generating pattern of a concave heat generating distribution in an internal side, but similar effects can be obtained also in a heater 110 shown in
Also the example 1 has shown a positioning of the heat generating patterns completely symmetrical in the direction of width of the substrate, but such configuration is not restrictive and effects of a certain level can be obtained also in a configuration that is not completely symmetrical in the direction of width (shorter side direction) of the substrate, as long as heat generating patterns of a same conductive path are positioned at an edge side and at the other edge side in the shorter side direction of the substrate while a heat generating pattern on the other conductive path is positioned therebetween in the shorter side direction of the substrate. Thus, a heater 120 as shown in
Also the first heat generating resistors are required to be present in at least two units, and may be present in three or more units. The second heat generating resistor is required to be present in at least one unit, and may be present in two or more units.
The effects of the example 1 can also be attained in a configuration of example 2 shown in the following.
The heat generating patterns 201a-1, 201a-2 are widened in the pattern width in plural steps from approximate center to both ends in the longitudinal direction, as in the example 1, to gradually reduce the resistance per unit length in the longitudinal direction, thereby constituting a convex type heat generation pattern. In the heat generating patterns 201a-1, 201a-2, a resistance per unit length in the longitudinal direction in the vicinity of a line α-α in
The heat generating pattern 201b is made narrower in the pattern width in plural steps from approximate center to both ends in the longitudinal direction to gradually increase the resistance per unit length in the longitudinal direction, thereby constituting a concave type heat generation pattern. In the heat generating pattern 201b, a resistance per unit length in the longitudinal direction in the vicinity of a line β-β in
The heat generating patterns 201a and 201b are set at a resistance of Ra=20 Ω (because of a parallel connection, Ra1=Ra2=40 Ω) and Rb=20 Ω, so that each heat generating pattern generates a power of 720 W under an application of 120 V. With such resistance setting, each heat generating pattern can be prepared with a same composition by selecting the pattern widths
Also as shown in
In the example 2, the relation between Wa1, Wa2 and Wb is different from that in the example 1. As the heat generating patterns 201a-1, 201a-2, formed on both edges sides of the heater substrate 204, are connected in parallel to constitute a single conductive path, in order to obtain a power same as in the example 1, each of the heat generating patterns 201a-1, 201a-2 has a resistance higher than in the example 1 (Ra1=Ra2=10Ω in example 1, and Ra1=Ra2=40Ω in example 2). It is therefore possible set Wa and Wb in
With the heat generating patterns 201a-1, 201a-2 formed on both edge sides in the direction of width of the substrate 204 have pattern widths Wa1, Wa2 narrower than those in the example 1, as in the case of parallel connection of the two first heat generating resistors in the present example, in case of a thermal uncontrollable state of the heater 200 by a failure of the triac 72a, the temperature elevation is suppressed in a central portion in the direction of width of the substrate but is promoted on both edge portions in the direction of width of the substrate to provide a thermal stress distribution as shown in
Also with the heat generating pattern 201b, formed inside the heat generating patterns 201a-1, 201a-2 has a pattern width Wb larger than that in the example 1, in case of a thermal uncontrollable state of the heater 200 by a failure of the triac 72b, the temperature elevation is suppressed in a central portion in the direction of width of the substrate but is promoted on both edge portions in the direction of width of the substrate to provide a thermal stress distribution as shown in
Table 1 summarizes results of verification in the examples 1 and 2 and in the comparative example, showing, in case of a thermal uncontrollable state of each of the convex type heat generating pattern and the concave type heat generating pattern with a power of 980 W, a maximum tensile stress after 3 seconds from the start of the uncontrollable, presence/absence of the heater breakage in the thermal uncontrollable (time of breakage in the absence of safety element 60) and presence/absence of the function of the safety element 60.
By connecting the heat generating patterns on both edge sides in the direction of width of the heater substrate, namely two first heat generating resistors, in parallel as in the example 2 to constitute a single conductive path, it is rendered possible to further reduce the tensile stress in a uncontrollable state in either heating generating pattern thereby increasing the margin to the heater breakage.
The effects of the examples 1 and 2 can also be attained in a configuration of example 3 shown in the following.
In the examples 1 and 2, there have been explained a fixing apparatus having a reference position of sheet passing at the center of the longitudinal direction and a heater provided therein. The present example 3 shows an embodiment of a fixing apparatus having a reference position of sheet passing provided at an end portion (longitudinal end) in the longitudinal direction (direction perpendicular to the conveying direction c of the recording material P), and a heater to be provided therein.
In the present example 3, the heat generating patterns 301a (301a-1, 301a-2) are widened in the pattern width in plural steps from a longitudinal end (sheet passing reference side S) toward the other end, to gradually reduce the resistance per unit length in the longitudinal direction, thereby gradually decreasing the heat generation amount, in case of a current passing, from a predetermined reference position in the longitudinal direction of the substrate 104, namely from the sheet passing reference side S, toward the other end. On the other hand, the heat generating pattern 301b is made narrower in the pattern width in plural steps to gradually increase the resistance per unit length in the longitudinal direction, thereby gradually increasing the heat generation amount, in case of a current passing, from the sheet passing reference side S, toward the other end.
The configuration of the present example 3 allows, in the fixing apparatus having a reference position of sheet passing at a longitudinal end, to reduce the thermal stress applied to the heater, thereby securing a margin to the heater breakage at a uncontrollable situation of the fixing apparatus. It is also possible to reduce a temperature elevation in a sheet non-passing area and to secure the fixing property at the same time, since the first heat generating resistors and the second heat generating resistor have different heat generating distributions.
The present invention is not limited to the examples 1-3 explained in the foregoing but is subject to any and all modifications within the technical concept of the invention.
For example, in the examples of the invention, a distribution in the heat generation in the longitudinal direction is formed by regulating the width of each heat generating pattern, but such distribution may also be formed by varying a thickness of the pattern or a composition of the material of the heat generating resistor in the longitudinal direction. Also the distribution of the heat generation in the longitudinal direction need not necessarily be a smooth change but can also be a stepwise changing distribution (
The present invention may also be applicable to a configuration in which the first heat generating resistors and the second heat generating resistor have different lengths in the heat generating resistor, thereby capable of switching the heat generating distribution of the heater (
Also a heater having three or more independent conductive paths can be realized within the technical concept of the invention (
Also the heater substrate is not limited to alumina but can be prepared with various ceramic materials such as aluminum nitride, and the heat generating pattern may be formed on either of a top surface and a bottom surface.
In the following there will be explained other examples of the present invention.
A heater substrate 20a is a laterally oblong thin plate member formed by a ceramic material having a heat resistance, a high thermal conductivity and an electrical insulating property, such as alumina or aluminum nitride.
The substrate 20a is provided with plural heat generating resistors 20b in substantially symmetrical manner with respect to the approximate center in the shorter side direction of the substrate.
The heat generating resistors 20b are constituted of a pair of main heat generating resistors 20b-1 (first heat generating resistors), and a pair of sub heat generating resistors 20b-2 (second heat generating resistors). The paired main heat generating resistors 20b-1 includes a heat generating resistor (20b-1-1) and a heat generating resistor (20b-1-2), which are provided in symmetrical positions with respect to the approximate shorter side center CL of the substrate. The paired sub heat generating resistors includes a heat generating resistor (20b-2-1) and a heat generating resistor (20b-2-2), which are provided in symmetrical positions with respect to the approximate shorter side center CL of the substrate. Each of the main and sub paired heat generating resistors 20b-1, 20b-2 is formed, on a surface of the substrate 20a, with a thickness of about 0.5 μm by printing and calcining a conductive thick film paste such as of Ag/Pd by a thick film printing method (screen printing method). In the direction of width (shorter side direction) of the substrate, the heat generating resistors at edge portions of the substrate constitute the main heat generating resistors while those at the central portion constitute the sub heat generating resistors, and each of the main and sub paired heat generating resistors is formed by connecting plural heat generating resistors in parallel. Also the electrodes on both electrical ends of the heat generating resistor (20b-1-1) and the heat generating resistor (20b-1-2) of the main paired heat generating resistors in symmetrical positions with respect to the approximate shorter side center CL of the substrate constitute common electrodes 22a, 22c. Also in the sub paired heat generating resistors, the electrodes on both electrical ends of the heat generating resistor (20b-2-1) and the heat generating resistor (20b-2-2) constitute common electrodes 22b, 22c. The common electrode 22c serves for both the main paired heat generating resistors and the sub paired heat generating resistors.
Each of the four heat generating resistors have a resistance of 18 Ω.
The temperature control means 27 is provided with a temperature detector 21, triacs 24 (24a, 24b) and a temperature controller (CPU) 23. The main power supply electrode 22a and the sub power supply electrode 22b of the main heat generating resistors 20b-1 the sub heat generating resistors 20b-2 are respectively connected to a triac 24a (first switching element) and a triac 24b (second switching element) for controlling an AC current from a commercial power supply 34. Also in series with the commercial power supply 34, there is connected a safety element (temperature fuse or thermo switch) 31 for preventing the excessive temperature elevation of the heater 20. The safety element 31 is positioned in contact with the heater 20 or in the vicinity thereof. The temperature controller controls the heater 20 at a predetermined temperature (target temperature) by controlling the on/off timing of the triacs 24a, 24b based on the temperature detected by the temperature detector 21, thereby controlling the current supply by the triac 24a to the paired main heat generating resistors 20b-1 between the main power supply electrode 22a and the common electrode 22c and the current supply by the triac 24b to the paired sub heat generating resistors 20b-2 between the main power supply electrode 22b and the common electrode 22c.
In the following there will explained a configuration of resistors in a heater 50 of a comparative example.
The heater 50 of the comparative example shown in
In the comparative example, as explained above, the main and sub heat generating resistors 50b-1, 50b-2 are divided in an edge side and another edge side in the shorter side direction of the substrate.
On the other hand, in the present example, in the paired main heat generating resistors (20b-1) and the paired sub heat generating resistors (20b-2), the heat generating resistors (20b-1-1, 20b-1-2) and those (20b-2-1, 20b-2-2) are respectively provided at an edge side and another edge side in the shorter side direction of the substrate, symmetric to the approximate shorter side center CL of the substrate. Stated differently, the two second heat generating resistors (20b-2-1, 20b-2-2) are provided, in the shorter side direction of the substrate, between the two first heat generating resistors (20b-1-1, 20b-1-2).
Comparison of the present example and the comparative example in
Also
The operation of the safety element 31 terminates the current supply to the main and sub heat generating resistors 20b-1, 20b-2, but, in this experiment, since the safety element 31 and the main and sub heat generating resistors 20b-1, 20b-2 are separately connected in this experiment, the power supply to the main and sub heat generating resistors 20b-1, 20b-2 is continued until the heater 20 is broken even after the function of the safety element 31.
As shown in
Therefore, even when the heater 20 causes a thermal uncontrollable (abnormal temperature elevation or overheating) by a failure in the temperature controller 23, the safety element is operated to terminate the current supply to the heat generating resistor before the heater is broken. It is thus possible to improve the durability and the reliability of the heater 20.
The effects of the heater 20 shown in
In
In the heater 20 shown in
Also depending on the design conditions, the heat generating resistors 20b may be constituted of three or more heat generating resistors. An example is shown in
For the heater 20 shown in
In the paired main heat generating resistors 20b-1 and the first and second paired sub heat generating resistors 20b-2, 20b-4, the main current supply electrode 22a and the sub current supply electrodes 22b, 22e are respectively connected with a triac 24a (first switching element), a triac 24b (second switching element) and a triac 24c (third switching element) are for controlling the AC current from the commercial power supply 34. Also the common electrode 22c is connected through the commercial power supply 34 through a safety element (temperature fuse or thermo switch in the present example) for preventing an excessive temperature elevation of the heater 20. The safety element 31 is positioned in contact with the heater 20 or in the vicinity thereof. The temperature controller 23 controls the on/off timing of the triacs 24a, 24b, 24c based on the temperature detected by the temperature detector 21. Thus it controls the heater 20 at a predetermined temperature (target temperature) by controlling the current supply by the triac 24a to the paired main heat generating resistors 20b-1 between the main power supply electrode 22a and the common electrode 22c, the current supply by the triac 24b to the paired sub heat generating resistors 20b-2 between the main power supply electrode 22b and the common electrode 22c, and the current supply by the triac 24c to the paired sub heat generating resistors 20b-4 between the main power supply electrode 22e and the common electrode 22c. Thus in the present example, between the two first heat generating resistors 20b-1-1 and 20b-1-2, there are provided two second heat generating resistors 20b-2-1, 20b-2-2, between which provided are the two third heat generating resistors 20b-4-1, 20b-4-2.
Also the heater 20 shown in
The heater 20 shown in
In
In the present example, no destruction occurs even in case the fixing apparatus 11 becomes by any reason incapable of controlling the current supply to the heater 20 whereby the electric power is continuously supplied to the heat generating resistor 20b of the AC line (primary circuit) to induce a thermal uncontrollable (abnormal temperature elevation or overheating) of the heater 20.
Since the heater 20 is not broken by the thermal uncontrollable, the safety element 31 such as a temperature fuse or a thermo switch inserted serially in the AC line is activated to open the AC line, whereby the power supply to the heat generating resistor 20b is intercepted and the thermal uncontrollable of the heater 20 is terminated.
The present example shows a configuration in which paired main heat generating resistors and a sub heat generating resistor are provided on top and rear surfaces of the ceramic substrate. Components equivalent to those in the example 4 are represented by same symbols and will not be explained further.
In the present example, in order to further improve the durability of the heater, paired main heat generating resistors 20b-1 and a sub heat generating resistor 20b-3 are provided symmetrically on top and rear surfaces of a ceramic substrate 21a. As shown in
In case the main heat generating resistors 20b-1 and the sub heat generating resistors 20b-3 on the top and rear surfaces of the substrate are connected in parallel, it is possible to adopt connections by through holes 22a-1, 22c-1, 22b-1 via the substrate 20a in the electrodes 22a, 22c, 22b corresponding to the respective heat generating resistors (cf.
In the present example, as the temperatures on the top and rear surfaces of the substrate 20a become approximately equal, the temperature distribution becomes always symmetrical to the approximate shorter side center CL even in a thick substrate 20a, whereby the thermal stress is canceled and is reduced drastically.
Referring to
Therefore, in the heater of the present example, the time to the heater breakage becomes longer because of a reduced thermal stress generating in the direction of thickness of the substrate (elimination of the uneven temperature distribution), and the operation time of the safety element becomes extremely short because it is positioned closer to the heat generating resistor. It is thus possible to secure a sufficient margin, even better than in the example 1. Thus, also the present example can improve the durability and the reliability of the heater.
In the present example, the safety element 31 such as a temperature fuse or a thermo switch inserted serially in the AC line is activated to open the AC line before the heater 20 is broken by the thermal uncontrollable, whereby the power supply to the heat generating resistor 20b is intercepted and the thermal uncontrollable of the heater 20 is terminated.
As the safety element 31 is activated to intercept the power supply before the heater 20 is broken by the thermal uncontrollable, it is rendered possible to reduce also current leaks in AC and DC lines, a breakage in the current leakage/temperature control systems, and an erroneous operation of a computer resulting from such current leakage.
Also since the heater 20 is not broken even at a maximum power, the resistance of the heat generating resistor can be selected low.
It is thus possible to provide an image forming apparatus capable of increasing the process speed, in case of employing the image heating apparatus as a fixing apparatus including a heating member.
(Others)
In the foregoing, the present invention has been explained by various examples and embodiments, but it will be readily understood to those skilled in the art that the principle and extent of the invention are not limited to the specified description and the drawings of the present specification but include various modifications and alterations within the scope of the appended claims.
This application claims priority from Japanese Patent Application Nos. 2004-182418 filed Jun. 21, 2004 and 2004-182419 filed Jun. 21, 2004, which are hereby incorporated by reference herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-182418 | Jun 2004 | JP | national |
| 2004-182419 | Jun 2004 | JP | national |
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