Image forming apparatus

A distance defined by expression 2 is secured between the exciting coil and the protection cover. This not only prevents a magnetic field with a specific magnetic field intensity or higher from leaking to the outside but also reduces the effect on the circuits in the apparatus or optionally installed circuits (including a printer controller and a FAX controller).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a fixing apparatus which fixes a developer image on paper and an image forming apparatus provided with the fixing apparatus, such as a copying machine or a printer.

2. Description of the Related Art

In the field of fixing apparatus installed in copying machines using electrophotographic processes, a fixing apparatus which includes an induction heating unit that generates hear by use of an electromagnetic induction has recently been put to practical use. In many cases, a heating (fixing) roller in which a heater is set and a pressure roller pressed against the heating roller at a specific pressure at one point of the outer circumference of the heating roller are used. It is well known that such a configuration enables not only the heat of the heating roller to be supplied to toner efficiently but also pressure for fixing the melted toner to a transfer medium to be applied to the transfer medium and toner efficiently.

One known fixing apparatus using induction heating is such that, for example, the magnetic flux leaking from the induction coil provided outside the fixing roller is suppressed by a shield member, which enhances the heat dissipation of the induction coil (Jpn. Pat. Appln. KOKAI Publication No. 2001-313162).

Another known induction heating fixing apparatus with exciting coil outside the rotating body is such that the arrangement of a magnetic material on the opposite side of the rotating body of the exciting coil not only increases the heat generation efficiency but also prevents the magnetic field produced from the exciting coil from leaking to the adjoining parts (Jpn. Pat. Appln. KOKAI Publication No. 11-297462).

A further known induction heating fixing apparatus is such that an induction heating member, a film member for moving the heating member, and an exciting coil fixing member have a ferromagnetic, high-sensitivity shield member, thereby preventing electromagnetic noise leaks (Jpn. Pat. Appln. KOKAI Publication No. 9-16006).

In an image forming apparatus with a heating unit using such induction heating, the high-frequency magnetic field generated from the coil can reach the circuit board in the apparatus or the printer controller or FAX controller optionally installed, which results in the problem of causing the devices or units to malfunction.

In addition, when a thin-film conductive member is used for induction heating to increase the efficiency in producing rapid temperature changes, or when the temperature is raised from room temperature to the requested fixing temperature (in warm-up), the high-frequency magnetic field generated from the coil can have a greater effect on the units in the apparatus.

Furthermore, even when a shield member is used, the high-frequency magnetic field from the coil can leak from the spacing or the like to secure a space through which the transfer medium is to be transported.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a fixing apparatus comprising: a coil which, when supplied with a voltage and current of a specific frequency, forms a first area having a magnetic field intensity equal to or higher than a specific intensity on the outside of the coil and a second area having a magnetic field intensity lower than the specific intensity on the outside of the coil; a conductive member which generates heat by a magnetic field supplied from the coil; and an protection cover covering the periphery of the fixing apparatus which is provided near the boundary line between the first area and the second area so that its surface may have the specific magnetic field intensity or less.

According to another aspect of the present invention, there is provided an image forming apparatus comprising: a fixing apparatus including a coil which, when supplied with a voltage and current of a specific frequency, forms a first area having a magnetic field intensity equal to or higher than a specific intensity on the outside of the coil and a second area having a magnetic field intensity lower than the specific intensity on the outside of the coil, and a conductive member which generates heat by a magnetic field supplied from the coil; and an protection cover housing the fixing apparatus which is provided in a position where the first area is formed inside so that its surface may have the specific magnetic field intensity or less.

According to a further another aspect of the present invention, there is provided an image forming apparatus comprising: a fixing apparatus including a coil which, when supplied with a voltage and current of a specific frequency, generates a magnetic field with a specific magnetic field intensity, and a conductive member which generates heat by a magnetic field supplied from the coil; and an protection cover covering the periphery of the fixing apparatus which is at a specific distance away from the surface of the coil.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, referring to the accompanying drawings, an example of an image forming apparatus to which an embodiment of the present invention is applied will be explained.

As shown inFIG. 1, an image forming apparatus (digital copying machine)101comprises an image reading unit (scanner)102for generating an image signal by reading an object (document) P to be read or copied, an image forming section103for forming an image on the basis of the image signal outputted from the scanner102, and an protection cover201provided on the outermost part of the image forming apparatus or between the units or circuits explained below. An image signal outputted from a printer board103bto which an interface103ais connected may be inputted to the image forming section103. An automatic document feeding unit (ADF)104may be provided integrally in the scanner102.

The image forming section103includes a fixing apparatus1, a photoreceptor drum105, a photolithography machine106, a developing machine107, a sheet cassette18, a pickup roller109, a transport path110, an aligning roller111, a discharge roller112, and a catch tray113.

The fixing apparatus1applies heat and pressure to a sheet Q which holds a toner image, thereby setting (fixing) the melted toner image to the sheet Q.

FIGS. 2 and 3are schematic diagrams to help explain an example of the fixing apparatus used in the image forming apparatus ofFIG. 1.

FIG. 2is a schematic plan view to help explain an example of the fixing apparatus1.

The fixing apparatus1includes a fixing (heating) roller2, a press (pressure) roller3, a pressure mechanism4, a peeling claw5, a temperature sensing element6, a cleaning member7, a heat generation abnormality sensing element8, a peeling claw9, a cleaning roller10, an exciting coil11, a coil holder12, a magnetic core13, a fixing protection cover301.

The heating roller2is such that a metal hollow cylinder conductive member with a thickness of about 1 mm, preferably about 0.5 mm is held in roller form. While in the embodiment, the conductive member of the heating roller12is made of iron, it may be made of stainless steel, nickel, aluminum, an alloy of stainless steel and aluminum, or the like. In the embodiment, a conductive member made of a iron (e.g., STKM material) tube of 0.4 to 1.5 mm thick is used for the heating roller2and the magnetic field from the exciting coil11is caused to pass through the heating roller2, thereby decreasing the magnetic field to a specific magnetic field intensity. On the surface of the heating roller2, a separate layer (not shown) is formed by depositing fluorocarbon resin, such as tetrafluoroethylene resin, to a specific thickness. In the embodiment, each of the fixing roller3and heating roller2has an outside diameter of 40 mm.

The pressure roller3is an elastic roller which is such that a rotation axis with a specific diameter is covered with silicone rubber, fluoric rubber, or the like of a specific thickness.

The pressure mechanism4is pressed against the axis line of the heating roller2at a specific pressure. The pressure roller3is kept almost in parallel with the axis line of the heating roller2.

As a result, a part of the outer circumferential surface of the heating roller3is deformed elastically, with the result that a specific nip is formed between the two rollers.

The heating roller2is rotated substantially a constant speed in the direction shown by the arrow by a fuser motor123explained later with reference toFIG. 4or a drum motor121for rotating the photoreceptor drum105. Since the pressure roller3is brought into contact with the heating roller2at a specific pressure by the pressure mechanism4, when the heating roller2is rotated, the pressure roller3is rotated in the position where it touches the pressure roller3, in the opposite direction to the direction in which the heating roller2is rotated.

The peeling claw5is provided on the circumference of the heating roller2on the downstream side in the direction in which the roller2is rotated by the nip where the heating roller2and the pressure roller3are in contact with each other and in a specific position near the nip. The peeling claw2peels the sheet Q passing through the nip from the heating roller2.

The temperature sensing element6, which is composed of, for example, a thermistor, senses the temperature of the outer circumferential surface of the heating roller2. The temperature sensing element6includes a temperature sensing element6aprovided almost in the middle in the longitudinal direction of the roller, and a temperature sensing element6bprovided at one end in the longitudinal direction of the roller. Two or more, for example, three temperature sensing elements6may be used.

The cleaning member7removes toner occasionally adhering to the fluorocarbon resin formed to a specific thickness on the outer surface of the heating roller2, and paper powder produced from sheets, and dust or the like floating in the apparatus and adhering to the heating roller2. The cleaning member7includes a cleaning member made of a material less liable to damage the fluorocarbon resin layer, such as a felt or fur brush, although it is in contact with the heating roller2, and a support member for supporting the cleaning member7. The cleaning member7may be rotated in contact with the surface of the heating roller2or be pressed against the outer circumferential surface of the heating roller2at a specific pressure (without rotation).

The heat generation abnormality sensing element8, which is, for example, a thermostat, senses the heat generation abnormality of the surface temperature of the heating roller2rising abnormally. When a heat generation abnormality has occurred, the heat generation abnormality sensing element8is used to cut off the electric power supplied to the heating coil (exciting coil) explained below.

The order in which the temperature sensing elements6a,6b, cleaning member7, and heat generation abnormality sensing element8are arranged and their locations are not limited to the order and locations shown inFIG. 2.

The peeling claw9for peeling the sheet Q from the pressure roller3and the cleaning roller10for removing toner adhered to the surface of the pressure roller3are provided on the circumference of the pressure roller3.

The heating roller2includes the exciting coil11for supply a specific magnetic field to the heating roller2composed of a conductive member, a coil holder12for holding the exciting coil11, and the magnetic core13for increasing the flux density of the magnetic field generated from the exciting coil11usable to cause the heating roller2to generate heat.

The coil holder12has high heat resistance and high insulation. For example, the coil holder is made of engineering plastic, ceramic, PEEK (polyether ether ketone) material, phenol material, unsaturated polyester, or the like.

The magnetic core13is made mainly of a material with low losses at high frequencies, such as a dust core. The excitation coil11may be an air-core coil without a magnetic core material.

The fixing protection cover301, which is the outermost part of the fixing apparatus1, holds the individual members included in the fixing apparatus in place.

FIG. 3is a schematic diagram of the fixing apparatus1ofFIG. 2viewed from the direction shown by the arrow R, with a part of the cover broken away.

The exciting coil11is composed of a first coil11alocated almost in the middle in the longitudinal direction of the heating roller2, and a second coil11band a third coil11clocated near at both ends in the longitudinal direction of the heating roller2, that is, at both ends of the first coil11a.

The first coil11a(center coil) is so formed that it has such a length as, when, for example, a A4-size sheet is conveyed in such a manner that its short side is in parallel with the axis line of the heating roller2, enables the width of the sheet contacting the outer circumferential surface of the roller2to be heated.

The second and third coils11b,11c(end coils) are a single coil electrically and connected in series. When they are arranged in line with the first coil11a as shown inFIG. 3, the longitudinal length is equal to the short side of an A3-size sheet.

The first, second, and third coils11a,11b, and11care made of a wire material whose cross-sectional area is equivalent to, for example, a 1-mm copper material. A stranded wire formed by stranding a plurality of thin wire materials with no insulating film, a litz wire formed by stranding a specific number of wire materials each covered with insulating material, or the like may be used as the wire material. Each of the coils11a,11b, and11ccan be formed by an arbitrary winding method. They are wound around the coil holder12.

A voltage and current of a specific resonance frequency are supplied to each coil. The coil then applies a magnetic field of a specific magnetic field intensity to a specific part of the heating roller2, thereby generating a magnetic flux and an eddy current in the heating roller2. The eddy current and heating roller resistance produce Joule heat, thereby heating the heating roller2.

Therefore, the second and third coils11b,11care helpful in heating the vicinities of both ends of the heating roller2, whereas the first coil11acan heat the middle in the longitudinal direction of the heating roller2.

The center coil and end coils may be divided, for example, almost in the middle of the heating roller2into two. Alternatively, for example, when a coil is provided for the pressure roller3, the first coil11a(center coil) may be provided on the heating roller2side and the second coil11b(end coil) may be provided on the pressure roller3side.

A wire material with a specific cross-sectional area is used for the first, second, and third coils11a,11b, and11c. Each of the first, second and third coils has a specific number of turns so as to resonate at its inherent resonance frequency, thereby maximizing its resistance value. They are designed to produce almost the same outputs. The total current output of the coils is sufficient to produce a magnetic flux capable of producing an eddy current to cause the heating roller2(or pressure roller3) to generate heat. This output is controlled by controlling the power consumed by the coils.

FIG. 4is a diagram to help explain a driving circuit for operating the fixing apparatus1shown inFIGS. 2 and 3and a control circuit for operating the image forming apparatus into which the fixing apparatus1is incorporated.

The heating roller2of the fixing apparatus1houses the exciting coil1(coils11a,11b,11c) for producing eddy current in the conductive material of the heating roller2as described above and thereby generating heat.

Connected to the exciting coil11is an exciting unit31for supplying high-frequency outputs of a specific frequency (current and voltage) to each coil of the exciting coil11.

The exciting unit31includes a switching circuit32capable of outputting high-frequency outputs to be supplied to the individual coils11a,11b,11cand a driving circuit33for inputting a specific control signal (the number of times of switching) to the switching circuit32to supply a specific output to the respective coils.

The switching circuit32is capable of, for example, connecting all of the coils11a,11b,11cin series, or connecting the coils11b,11cin series and then connecting the resulting series connection in parallel with the coil11a, or connecting all of the coils11a,11b,11cin parallel. That is, the switching circuit32also functions as a selector unit capable of setting a series connection or a parallel connection between the individual coils11a,11b,11c.

A direct-current voltage obtained by rectifying a received commercial power alternating voltage with a rectifier (not shown) is supplied via the driving circuit33to the switching circuit32.

At this time, the driving circuit33informs the switching circuit32the high-frequency outputs to be outputted by the switching circuit32, or the time that the switching elements (not shown) are turned on for the respective coils11a,11b,11cto output the coil outputs, specific heating power, or the number of times the switching element is turned on during a unit time (driving frequency).

In the embodiment, the driving circuit33informs the switching circuit32to supply a first frequency f1to the coil11aand a second frequency f2to the coil11b. In other words, the magnitude of the magnetic flux, or the heating power, outputted from each coil to produce an eddy current in the heating roller2to raise the temperature of the heating roller2can be set to an arbitrary magnitude by controlling the driving circuit33to change the outputs from the switching circuit32to the respective coils.

The heating power is generally managed in values in the form of the amount of power consumed by each coil. Hereinafter explanation will be given, regarding the coil output (power consumption) of each coil just as an electric power inputted to a coil and the frequency of the power consumption as the frequency used.

The electric power supplied from the rectifying circuit to an arbitrary one or all of the coils is always monitored by an electric power sensing circuit41provided in a specific place, such as between the rectifying circuit and the input terminal of a commercial power supply, between the rectifying circuit and the driving circuit33, or between the driving circuit33and the switching circuit32.

The result of the monitoring by the electric power sensing circuit41is fed back to the driving circuit33with a specific timing. To make it possible to sense the burnout or the like of the driving circuit33, the output of the electric power sensing circuit41is also inputted to a main control unit151on the image forming section103side.

The main control unit151is connected to a motor driving circuit153.

The motor driving circuit153is connected to a main motor121for supplying driving force to a specific member of the image forming section103, such as the photoreceptor drum105, and the fuser motor123for rotating the heating roller2.

Next, an example of control to raise the temperature at the outer circumferential surface of the heating roller2to a specific temperature will be explained.

As is commonly known, in the fixing apparatus1of the induction heating type, each of the coils11a,11b,11cof the exciting coil11produces a magnetic flux in a specific direction, depending on the amount of power supplied to the coil and the form of the coil. Therefore, eddy current develops in the metal part of the heating roller2so as to prevent the change of the magnetic field produced by the magnetic flux generated in the coil. As a result, Joule heat is caused in the metal part of the heating roller2by the eddy current and the resistance of the metal part.

The heating roller2generates heat due to the Joule heat, thereby raising the temperature of the heating roller2, with the result that the sheet Q passing through between the heating roller2and the pressure roller3is heated. In normal heating whereby the whole area in the longitudinal direction of the heating roller2is heated almost uniformly, the switching circuit32explained inFIG. 4supplies high-frequency outputs (current and voltage) to each of the coils11a,11b,11c.

In a case where electric power of a specific frequency is supplied to a first coil and a second coil differing in a coil constant, such as inductance L (the inductance of the second coil is lower than that of the first coil), when an independent switching circuit is provided, an attempt to control the output of the second coil in the same range as that of the first coil requires the second coil to have the frequency range of about 30 kHz to 40 kHz, provided that, for example, the frequency range required to control the output of the first coil in the range of 1 kW to 600 W is from 20 kHZ to 30 kHz.

That is, when the coil outputs of the coils differing in inductance are changed, operating the individual coils independently results in a small variation in the frequency.

In contrast, in a case where the first coil with a specific inductance and the second coil with a lower inductance than that of the first coil are connected to a single switching circuit and electric power of a specific frequency is supplied, for example, when the output of the first coil is 900 W and the output of the second coil is 1.1 kW at a frequency of 20 kHz, the output of the first oil is changed to 500 W and the output of the second coil is changed to about 0.9 kW at a frequency of 30 kHz. In addition, when the frequency is changed to 40 kHz, the output of the first coil is lowered to about 200 W, whereas the output of the second coil is kept at about 500 W.

Next, the relationship between the frequency of electric power supplied to each coil and the coil output will be explained.

For example, electric power supplied to each of the coils11a,11b,11ccan be changed in the range of, for example, 700 W to 1.5 kW arbitrarily in terms of the amount of power consumed by the coil. As is generally known, the amount of current flowing in any one of the coils11a,11b,11cof the exciting coil11is determined by setting a frequency applied to the coil, an impedance, and so forth.

For example, in a case where electric power differing only in frequency is supplied, even when the current value is 10 mA, the inductance and pure resistance are changed as follows: inductance L=24.6 μH, pure resistance R=1,2 Ω at 25 kHz, inductance L=18.69 μH, pure resistance R=3.5 Ω at 100 kHz, and inductance L=15.1 μH, pure resistance R=4,9 Ω at 1 MHz. Therefore, the higher the frequency, the larger the impedance.

As described above, the exciting coil11used for induction heating generates a magnetic field of a different intensity according to the frequency of the electric power supplied (frequency used). Therefore, in an operation mode requiring a high magnetic field intensity, even when the exciting coil11is housed in the heating roller2and therefore the heating roller2decreases the magnetic field intensity, there is a possibility that the magnetic field might leak to the outside. In addition, in a compact image forming apparatus, or in an image forming apparatus which has the catch tray113ofFIG. 1ofFIG. 1between the cassette108and the scanner102, the fixing apparatus1is provided very close to the outside of the apparatus, there is a possibility that the magnetic field from the exciting coil11might leak to the outside, from a structural viewpoint.

If the magnetic field from the exciting coil11leaks to the outside, problems arise. They include malfunctions of the circuits or an optionally installed printer controller, FAX controller, or the like provided near the fixing apparatus1, and an adverse effect of electromagnetic waves on users or servicepersons.

FIRST EMBODIMENT

FIG. 5is a view of adjoining parts of a fixing apparatus provided in the image forming apparatus ofFIG. 1, showing the positional relationship between the outside wall (protection cover) of the image forming apparatus101and the fixing apparatus.

As shown inFIG. 5, in the image forming apparatus101which has a catch tray113as shown inFIG. 1between the cassette108and the scanner102, the fixing apparatus1is provided close to the outside. Specifically, the fixing apparatus1is enclosed by an protection cover201aclose to the unit1on the upper side, an protection cover201bclose to the unit1on the right side, and an protection cover201cclose to the unit1on the left side. The protection covers201a,201b,201care included in the protection cover201of the image forming apparatus101. Although they are divided and indicated by reference numerals for the sake of explanation, they may be formed integrally out of the same material.

Although not shown, a specific circuit or an optionally installed circuit or the like may be provided above the protection cover201a. The protection cover201bis the outermost part of the image forming apparatus shown inFIG. 1.

Therefore, the magnetic field from the exciting coil11leaking to the outside of the protection covers201a,201bis required to have such a strength as has no effect on the nearby circuits, optionally installed units, including a printer controller and a FAX controller, the user or serviceperson, and so forth.

In the image forming apparatus101ofFIG. 1, since the catch tray113is formed on the right side of the protection cover201c, the effect of the magnetic field need not be taken into account.

The fixing apparatus1is provided in a position that secures at least a distance of d1between the surface of the exciting coil11and the protection covers201a,201bso that the magnetic field intensity at the surface of the protection covers201a,201bmay be at a specific value or less. The position that secures the distance d1presents a magnetic field intensity of t1at which the effect on the nearby circuits and the like is alleviated.

The distance d1is defined as a point on a rough circle M1, the boundary line between a first area L1with a magnetic field intensity equal to or higher than the specific magnetic field intensity t1and a second area L2with a magnetic field intensity lower than the specific magnetic field intensity t1, when a voltage and current of a specific frequency of F are supplied. It is desirable that the specific frequency F should have the frequency (e.g. 20 kHz) of the electric power supplied to the exciting coil to generate a magnetic field of the highest magnetic field intensity. When the fixing apparatus is being used, the magnetic field intensity at the surface of the protection cover can be made t1or lower.

As known from “Radio Wave Protection Standard” or the like, when a frequency of f (kHz) in the range of 0.8<f<150 is used, if a magnetic field with a magnetic field intensity of 6.25 μT or more is applied to a circuit or the like, a problem might arise: for example, the circuit will possibly malfunction. Therefore, a circuit is provided around the unit generating a magnetic field, the magnetic field intensity applied to the circuit is required to be lower than 6.25 μT.

Therefore, each of the surfaces of the protection covers201a,201bhas to be provided in a position where the magnetic field intensity is lower than 6.25 μT. Thus, it is desirable that the magnetic field intensity t1should be equal to or lower than 6.25 μT.

In the first embodiment, the intensity of the magnetic field leaking from the exciting coil11to the adjacent areas is measured using a Combinover MPR-II (the frequency range of 2 k to 400 kHz). From the result of the measurement, the distance d1is defined. Since the exciting coil11might generate harmonic magnetic fields according to the frequency of the voltage and current applied (the frequency used), it is desirable that the frequency range of the measuring instrument should be at least five times or more as high as the frequency used.

As known from “Method of checking conformance to the Radio Wave Protection Standard (ARIBRT-11),” it is determined that the range where the effect on the human body having the measuring instrument or the nearby metals or the like is alleviated in measuring the magnetic field shall be such that the distance from the measuring instrument is 20 cm or more away from every object in the case of a radiation source of a frequency of 300 MHz or more.

If the distance from the measuring instrument to one of the protection covers201a,201bis D1and the magnetic field intensity measured by the instrument at the position is T1, since the magnetic field intensity is inversely proportional to the square of the distance, the distance d1from the surface of the exciting coil11to the protection covers201a,201bmeets the following equation:

Thus, the distance d1has the range defined by the following expression:

As a result of the measurement under the above conditions, when the generated magnetic field intensity was the highest, for example, when the power consumption of the coil at the time of warm-up was the largest (e.g. 1300 W), the frequency was 20 kHz and the magnetic field intensity at a point 30 cm away from the protection cover was 350 nT.

Substituting the result into equation 2 gives the following:

6250=350⁢(30+d1)d1(Equation⁢⁢3)
Then, it follows that d1=9.30.

Therefore, it is desirable that the exciting coil11should be provided at least 10 cm or more away from the protection covers201a,201b.

As a result, the magnetic field intensity applied to the circuits or optional units or the like provided near the fixing apparatus1is alleviated, which reduces malfunctions of the nearby devices.

It goes without saying that a similar effect can be expected by placing no object liable to be affected by the magnetic field in the first area L1defined by the distance d1in a position other than the protection covers201a,201b, for example, in the areas at both ends of the heating roller2in the axis direction, or in the lower area of the fixing apparatus1.

SECOND EMBODIMENT

The image forming apparatus shown inFIG. 1enables a magnetic field intensity of 6.25 μT or less to be obtained at the surface of a fixing protection cover301provided inside the protection covers201a,201b,201cand in the outmost part of the fixing apparatus1as shown inFIG. 6.

As shown inFIG. 6, the fixing protection cover301is provided a distance of d2or more away from the surface of the exciting coil11excluding the inlet Qin and outlet Qout for the sheet Q. Like the distance d1, the distance d2is defined as a position at which the effect on the nearby circuits and the like of the magnetic field intensity of t1is alleviated. Specifically, the distance d2is defined as a point on a rough circle M2, the boundary line between a first area L3with a magnetic field intensity equal to or higher than the specific magnetic field intensity t1and a second area L4with a magnetic field intensity lower than the specific magnetic field intensity t1, when a voltage and current of a specific frequency of F are supplied. It is desirable that the specific frequency F should have the frequency (e.g. 20 kHz) of the electric power supplied to the exciting coil to generate a magnetic field of the highest magnetic field intensity.

As described above, the specific magnetic field intensity t1is a magnetic field intensity that has little effect on the nearby circuits. Equation 2 also holds for d2.

Therefore, the fixing protection cover301is provided at least 10 centimeters away from the surface of the exciting coil11.

As a result, even when the circuits and optional units and the like are provided next the fixing protection cover301inside the protection covers201a,201b,201c, the devices provided nearby can be prevented from malfunctioning.

Since the fixing apparatus where the distance from the surface of the exciting coil11to the fixing protection cover301is equal to d2or more needs no fixed position set in the image forming apparatus in which the unit is to be installed, the labor involved in manufacture can be reduced. In addition, even when the fixing apparatus has been installed in an apparatus other than the image forming apparatus designed in manufacture owing to failure of the fixing apparatus or the like, the magnetic field intensity outside the fixing protection cover301of the fixing apparatus1can be secured at 6.25 μT or less.

FIG. 7is a schematic diagram to help explain another example of an induction-heating fixing apparatus used in the image forming apparatus ofFIG. 1.

The fixing apparatus401includes a conductive film402, a pressure roller403, an exciting coil404, a peeling claw405, a temperature sensing element406, a cleaning member407, and a heat generation abnormality sensing element408. Although not shown, the fixing apparatus401is installed in an image forming apparatus having the same function as that of the image forming apparatus ofFIG. 1. The fixing apparatus401is provided near an protection cover409. The protection cover409is the outermost part of the image forming apparatus provided with the fixing apparatus401.

The conductive film402, which is an endless belt made of metal, such as nickel or stainless steel of several tens of micrometers in thickness, is moved in the direction shown by the arrow by a roller member provided at a specific position inside. The conductive film402may be a metal film obtained by depositing metal to a specific thickness on the surface of a highly heat-resistant resin film to form a sheet material and forming the sheet material into an endless belt shape.

The pressure roller403applies a specific pressure to the conductive film402, thereby forming a nip with a specific width. Since the width of the nip can be determined arbitrarily by, for example, selecting the width in the direction of the movement of the conductive film402, secured by the two roller members as shown inFIG. 7, a much greater nip width can be realized easily. In addition, the pressure roller403is rotated by a driving motor (not shown) in the direction shown by the arrow, that is, in the direction in which its surface is moved in the same direction at the contact position as that of the movement of the conductive film402.

The exciting coil404, which is provided at a specific position outside the conductive film402, applies a specific magnetic field to the outer circumferential surface of the conductive film402.

An eddy current flows in the conductive film402to which a magnetic field has been applied, with the result that Joule heat is generated. The sheet Q passes through the nip formed between the conductive film402and the pressure roller403, thereby applying specific heat and pressure to the sheet on which the toner image is held, with the result that an image is formed on the sheet. The sheet Q passed through the nip is peeled by the peeling claw405provided on the downstream side of the pressure roller403and then is conveyed to the catch tray113. The attached toner, paper powder or dust, and the like are removed by the cleaning member407provided on the downstream side of the nip at the outer circumferential surface of the conductive film402. The temperature sensing element406and heat generation abnormality sensing element408are provided near the outer circumferential surface of the conductive film402and connected to the respective specific positions of the electric circuit shown inFIG. 4, which enables the conductive film402to be kept at a specific temperature.

The fixing apparatus401is provided near the protection cover409shown on the right. The protection cover409is connected to the outside on the right.

The fixing apparatus401is provided so as to leave at least a distance of d3between the surface of the exciting coil404and the protection cover409so that the magnetic field intensity at the surface of the protection cover409may be equal to or lower than a specific value. Like d1, the distance d3is defined as a point on a rough ellipse M3, the boundary line between a first area L5with a magnetic field intensity equal to or higher than the specific magnetic field intensity t1and a second area L6with a magnetic field intensity lower than the specific magnetic field intensity t1, when a voltage and current of a specific frequency of F are supplied. It is desirable that the specific frequency F should have the frequency (e.g. 20 kHz) of the electric power supplied to the exciting coil to generate a magnetic field of the highest magnetic field intensity.

As previously described, the specific magnetic field intensity t1is a magnetic field intensity that has little effect on the nearby circuits. Equation 2 also holds for d3.

Therefore, it is desirable that the protection cover409should be provided at least 10 centimeters away from the surface of the exciting coil404.

Unlike the fixing apparatus1having the exciting coil11inside the heating roller2, the exciting coil404is provide outside the conductive film402. Therefore, since the magnetic field intensity is not decreased by the conductive film402, d3larger than d1determined using equation 3 is required.

Therefore, the required d3may be secured between the surface of the exciting coil404and the protection cover409by providing the exciting coil404in a position further away from the protection cover409, that is, on the left side of the conducive film402.

FOURTH EMBODIMENT

FIG. 8is a schematic diagram to help explain a further another example of an induction-heating fixing apparatus used in the image forming apparatus ofFIG. 1.

The fixing apparatus501includes a conductive film502, a pressure roller503, an exciting coil504, a temperature sensing element505, a cleaning member506, and a heat generation abnormality sensing element507. Although not shown, the fixing apparatus501is installed in an image forming apparatus having the same function as that of the image forming apparatus ofFIG. 1. The fixing apparatus501is provided near an protection cover508. The protection cover508is the outermost part of the image forming apparatus provided with the fixing apparatus501.

Unlike the conductive film502, which is an endless belt or a metal film comprising a metal layer, such as nickel or stainless steel of several tens of micrometers in thickness, is moved in the direction shown by the arrow by a roller member provided at a specific inside position.

The pressure roller503applies a specific pressure to the conductive film502, thereby forming a nip with a specific width. The pressure roller503is rotated by a driving motor (not shown) in the direction shown by the arrow, that is, in the direction opposite to the direction in which the conductive film502rotates.

The exciting coil504, which is provided at a specific position inside the conductive film502, applies a specific magnetic field to the conductive film502.

The fixing apparatus501is provided near the protection cover508shown on the right. The protection cover409is connected to the outside on the right.

The fixing apparatus501is provided so as to leave at least a distance of d4between the surface of the exciting coil504and the protection cover508so that the magnetic field intensity at the surface of the protection cover508may be equal to or lower than a specific value. Like d1, the distance d4is defined as a point on a rough circle M4, the boundary line between a first area L7with a magnetic field intensity equal to or higher than the specific magnetic field intensity t1and a second area L8with a magnetic-field intensity lower than the specific magnetic field intensity t1, when a voltage and current of a specific frequency of F are supplied. It is desirable that the specific frequency F should have the frequency (e.g. 20 kHz) of the electric power supplied to the exciting coil to generate a magnetic field of the highest magnetic field intensity.

As previously described, the specific magnetic field intensity t1is a magnetic field intensity that has little effect on the nearby circuits. Equation 2 also holds for d4.

As described above, in the present invention, the distance defined by equation 2 is secured between the exciting coil and the protection cover, thereby preventing a magnetic field of a specific magnetic field intensity or higher from leaking to the outside, which alleviates the effect on the circuits in the apparatus or optionally installed circuits (including a printer controller and a FAX controller). The invention may be applied to apparatuses other than those explained in the embodiments.

Preventing the high-frequency magnetic field generated from the coil from leaking to the outside of the fixing apparatus makes it possible to prevent the other devices in the apparatus from malfunctioning.

In addition, even when a magnetic field differing in intensity is generated according to the operation mode, the effect on the other devices in the apparatus can be reduced to a minimum.