Fixing Unit and Heat Roller for Fixing Unit

The present invention relates to a fixing unit and a heat roller for the fixing unit as used in an electrophotographic image forming process.

Conventional image forming devices, such as laser beam printers, copiers, and the like, employ an electrophotographic image forming process in which a toner image is electrophotographically transferred to a recording sheet, and then the toner image is fixed to the recording sheet at a fixing unit by applying heat to the recording sheet using a heat roller.

A conventional fixing unit includes a heat roller, a pressing roller biased towards the heat roller, a drive mechanism for rotating the heat roller and the pressing roller, and an electrical system for providing power to the heat roller so that the heat roller can generate heat.

The heat roller operates at a predetermined fixing temperature (e.g., 140.degree. C.) and is provided with, for example, a resistance heating member to heat the heat roller to the predetermined fixing temperature. Conventionally, the resistance heating member is provided inside the heat roller and electrodes are provided so that a voltage can be applied to the resistance heating member to cause the resistance heating member to generate heat.

However, due to the position of the heat roller in the imaging forming device, the feeding of the paper, or the like, certain portions of the heat roller may overheat or cool more quickly than other portions of the heat roller causing an uneven temperature distribution along the axial direction of the heat roller. If portions of the heat roller overheat, elements of the image forming device that are made from synthetic resin or the like and that are located around the heat roller may be deformed, or, in the worst case, the elements or recording sheet may catch on fire. If portions of the heat roller are cooler than the required fixing temperature, the fixing process may not be properly performed, providing a lower quality image.

As an example, typically, axial end portions of the heat roller radiate heat more quickly than a central portion thereof. Thus, the temperature at the end portions tends to be lower than the temperature at the central portion. Thus, in order to obtain an even temperature distribution along the axial direction of the heat roller, it is necessary that the end portions be heated more than the central portion. This problem may be overcome by, for example, varying the thickness of the resistance heating member along the axis of the heat roller or using a different material for the resistance heating member at the end portions of the heat roller, however, such solutions complicate the manufacturing of the resistance heating member and thus may increase the manufacturing cost.

A second example of uneven temperature distribution occurs when a recording sheet having a smaller width is fed through the fixing unit. In this case, the portion of the heat roller that does not contact the recording sheet may overheat, since heat is not drawn away from this portion of the heat roller by the recording sheet. As mentioned above, overheating may harm the image forming device. This uneven temperature distribution cannot be as easily corrected by the methods discussed above for correcting uneven heat radiation at the axial ends of the heat roller.

Japan Patent Provisional Publication SHO 62-279378, discloses a fixing unit including a heat roller using a resistance heating member. The heat roller is provided, on an inner side thereof, with a resistance heating member that includes an insulating layer and a conductive layer. Further, the fixing unit includes a pair of circular electrodes, that contact the inner circumference of the resistance heating member, and an electrode drive mechanism for driving the electrodes along the axis of the heat roller. A controller controls the electrode drive mechanism to adjust the positions of the brush members along the axis of the heat roller such that electricity is passed only through a portion of the resistance heating member which is positioned between the brush members, corresponding to the width of the recording sheet.

However, the above arrangement, i.e., the use of the circular electrodes and an electrode drive mechanism to move the electrodes relative to the resistance heating member, involves a complicated and expensive driving and control system.

It is therefore an object of the present invention to provide an improved fixing unit and heat roller for fixing unit which allow easy adjustment of the temperature distribution along the axis of the heat roller.

According to one aspect of the invention there is provided a fixing unit that includes a base, a heat roller rotatably supported on the base, and a heating member driving unit electrically connected to the heat roller.

The heat roller includes a hollow cylindrical tube, an insulating layer affixed to an inner side of the tube, a plurality of electrode portions formed on the insulating layer extending around an inner circumference of the tube, a plurality of electrodes, corresponding to the plurality of electrode portions, fixed to the base and slidably contacting the corresponding plurality of electrode portions, and a plurality of resistance heating patterns formed on the insulating layer. The resistance heating patterns are arranged along an axial length of the tube and each of the resistance heating patterns is connected to predetermined electrode portions of the plurality of electrode portions.

The heating member driving unit selectively applies a voltage to the plurality of electrodes.

With this arrangement of the fixing unit, the plurality of resistance heating patterns can be selectively energized to heat only selected portions of the heat roller along the axial length thereof and both the problems of differing heat radiation levels and overheating of areas of the heat roller due to differing recording sheet widths can be compensated for by varying the amount of heating applied to different axial portions of the heat roller.

In a particular case, the heating member driving unit includes at least one voltage source, a plurality of switch circuits that connect the voltage source with at least a selected electrode of the plurality of electrodes, and a controller for controlling the plurality of switch circuits according to predetermined data.

The predetermined data may be, for example, representative of the size of a recording sheet or the like. In this way the controller may control the switches in order to control the heating of the heat roller according to the predetermined data.

In another particular case, each of the plurality of electrodes may include a support electrode extending from the base and a brush supported by the support electrode which slidably contacts the corresponding electrode portion. In this way, a circular electrode is not required.

The plurality of resistance heating patterns may be arranged according to the requirements of the fixing unit. For example, the plurality of resistance heating patterns may be arranged having substantially equal lengths along the axial length of the tube or may be arranged having different lengths along the axial length of the tube.

According to another aspect of the invention, there is provided a heat roller that includes a hollow cylindrical tube and a resistance heating member provided inside the tube for selectively heating selected axial lengths of the tube.

In particular, the resistance heating member includes an insulating layer, at least three electrode portions, and a plurality of resistance heating patterns. The at least three electrode portions are formed on said insulating layer to extend in a circumferential direction of said tube and the plurality of resistance heating patterns are formed on said insulating layer and arranged along an axial length of said tube. Further, the ends of each of said resistance heating patterns are connected to predetermined support electrodes of said at least three electrode portions.

With this arrangement, a voltage may be applied to a particular resistance heating pattern to heat only the portion of the heat roller near the particular resistance heating pattern. As above, both the problems of differing heat radiation levels and overheating of areas of the heat roller due to differing recording sheet widths can be compensated for by varying the amount of heating applied to different axial portions of the tube.

In this embodiment, the plurality of resistance heating patterns may be electrically connected in series, such that, by applying a voltage across all of the plurality of resistance heating patterns, the whole heat roller may be heated.

In a particular case, each consecutive resistance heating pattern is connected to each previous resistance heating pattern at a connecting portion and each of said connecting portions is connected to a corresponding one of said at least three electrode portions.

Further, if each of the connecting portions are electrically connected to a single electrode portion, the number of electrode portions required may be reduced.

As above, the plurality of resistance heating patterns may be arranged according to the requirements of the heat roller. For example, the plurality of resistance heating patterns may be arranged having substantially equal lengths along the axial length of the tube, having different lengths along the axial length of the tube, or symmetrically with respect to a center point along the axial length of the tube.

Preferably, the plurality of resistance patterns are formed as serpentine patterns to uniformly distribute heat over a predetermined area.

As a particular example, the tube may be formed of aluminum to provide good heat conduction, the insulating layer may be formed of resin to provide good heat conduction and sufficient electrical insulation, and the electrode portions and the resistance heating patterns may be formed of stainless steel to provide sufficient resistance heat formation.

Further preferably, the heat roller further includes a deposit prevention layer provided on an outer surface of the tube in order to prevent toner from adhering to the outer surface of the heat roller.

In a particular case, a dimension of the insulating layer corresponding to an inner circumference of the tube is slightly longer than a dimension of the inner circumference, such that, when the insulating layer is affixed to the tube, a top portion of the insulating layer overlaps a bottom portion of the insulating layer.

Since the insulation layer completely covers the inner surface of the tube, there is no danger that electrical brushes or connectors used for making electrical contact with the electrode portions will come in contact with the tube. Further, if the top portion is arranged to overlap on the bottom portion in a direction opposite to that in which the tube rotates during operation of the heat roller, there is less chance that the electrical brushes or connectors will be interfered with as the tube rotates. This arrangement further allows a connector to run through the bottom portion of the insulating layer to connect the resistance heating patterns to the electrode portions without interfering with the resistance heating patterns or the electrical brushes or connectors.

FIG. 1 is a schematic perspective view of a fixing unit according to a first embodiment of the invention;

FIG. 2 is an enlarged partial sectional view of a heat roller of the fixing unit of FIG. 1;

FIG. 3 is a developed view of a resistance heating member of the heat roller;

FIG. 4 is a sectional side view of the heat roller of FIG. 1;

FIG. 5 is a developed view of an alternative resistance heating member;

FIG. 6 is a developed view of a resistance heating member according to a second embodiment of the invention;

FIG. 7 is a schematic diagram of an electrical system for the fixing unit according to the first embodiment;

FIG. 8 is a schematic diagram of an alternative electrical system for the fixing unit according to the first embodiment; and

FIG. 9 is a schematic diagram of an electrical system for the fixing unit according to the second embodiment.

A fixing unit according to an embodiment of the invention is described as applied to an image forming device such as a copier, a printer, a facsimile device, or the like.

FIG. 1 is a schematic perspective view of a fixing unit 100 according to an embodiment of the invention. The fixing unit 100 includes a base (not shown), a heat roller 10, a pressing roller 20, and a heating member driving unit 50.

The heat roller 10 and the pressing roller 20 are rotatably supported by the base such that the heat roller 10 and the pressing roller 20 are biased towards each other. In operation, the heat roller 10 is heated and the heat roller 10 and the pressing roller 20 are rotated to feed a recording sheet P, bearing an unfixed toner image, between the heat roller 10 and the pressing roller 20 in the direction indicated by an arrow A in FIG. 1. As the recording sheet P is fed, the toner image on the recording sheet P is fixed onto the recording sheet P by heat from the heat roller 10.

As shown in the enlarged sectional view of FIG. 2, the heat roller 10 includes a roller body 11, a deposit prevention layer 12 and a resistance heating member 13.

The roller body 11 is formed as a hollow cylindrical tube made of a material having excellent heat conductivity, such as aluminum or the like.

The deposit prevention layer 12 is formed on an outer surface of the roller body 11 in order to prevent toner from adhering to the heat roller 10. In particular, the deposit prevention layer 12 may have a thickness of approximately 10 to 20 .mu.m, and is preferably formed from a material having excellent heat resistance, such as a fluoride resin (e.g., a fluoride resin sold under the Trademark "TEFLON", or the like.

The resistance heating member 13 is provided on an inner surface of the roller body 11. The resistance heating member 13 includes a flexible insulating layer 14 and a flexible conductive layer 15. The insulating layer 14 may be made from, for example, a polyimide resin having a thickness of 30 to 50 .mu.m. The conductive layer 15 may be formed as, for example, a stainless steel foil having a thickness of 30 to 50 .mu.m adhered to the insulating layer 14. Further, the conductive layer 15 is formed having a predetermined pattern as described with reference to FIG. 3.

FIG. 3 is a developed view of the resistance heating member 13 showing the insulating layer 14 and the conductive layer 15. The axial direction of the heat roller 10 is indicated as an X direction and the inner circumference direction of the heat roller 10 is indicated as a Y direction. FIG. 4 is a side sectional view of the heat roller 10 with the resistance heating member 13 installed therein. In FIG. 4, the dimensions of the deposit prevention layer 12, the insulating layer 14, and the conductive layer 15 are exaggerated for clarity.

As shown in FIGS. 3 and 4, the insulating layer 14 is longer in the Y direction that the inner circumference of the roller body 11, such that, when the resistance heating member 13 is installed in the roller body 11, a top area 14b will overlap and cover a bottom area 14a.

As shown in FIG. 3, the conductive layer 15 includes a resistance heating portion 15a, and first, second, and third electrode portions 15b, 15c, and 15d.

The first electrode portion 15b and the second and third electrode portions 15c and 15d are formed at opposite ends of the insulating layer 14 (i.e., the right and left sides of FIG. 3) extending in the Y direction but not entering on the bottom area 14a. The third electrode portion 15d is formed closer to the end of the insulating layer than the second electrode portion 15c, i.e., to the right of the second electrode portion 15c in the view of FIG. 3.

The resistance heating portion 15a includes first and second resistance heating patterns, referred to as first and second serpentine patterns 15e and 15f, each arranged over approximately half the X direction of the insulating layer 14 and over all of the Y direction of the insulating layer 14 except the bottom area 14a, i.e., the first serpentine pattern 15e is arranged on the left half of the insulating layer 14 and the second serpentine pattern 15f is arranged on the right half of the insulating layer 14, as shown in the view of FIG. 3. The first serpentine portion 15e includes first and second end portions 151a and 151b and, similarly, the second serpentine portion 15f includes first and second end portions 152a and 152b.

The second end portions 151b and 152b of the first and second serpentine patterns 15e and 15f are electrically connected together near the bottom area 14a. An electrical connector 15g is connected at the connection of the second end portions 151b and 152b, runs along the bottom area 14a, and is connected to the third electrode portion 15d. Further, the first end portion 151a of the first serpentine pattern 15e is electrically connected to the first electrode portion 15b in the top area 14b and the first end portion 152a of the second serpentine pattern 15f is electrically connected to the second electrode portion 15c in the top area 14b.

Thus, if a voltage is applied across the first electrode portion 15b and the second electrode portion 15c, current flows through both the first and second serpentine patterns 15e and 15f, since the first and second serpentine patterns 15e and 15f are connected in series. If a voltage is applied across the first electrode portion 15b and the third electrode portion 15d, current flows only in the first serpentine pattern 15e. Similarly, if a voltage is applied across the second electrode portion 15c and the third electrode portion 15d, electric current flows only in the second serpentine pattern 15f.

The resistance heating member 13 may be formed by, first, bonding a conductive material, for example, a metal foil (stainless steel, copper foil, or the like), onto the insulating layer 14, and then, screen printing a resist (not shown), corresponding to the resistance heating portion 15a and the first, second, and third electrode portions 15b, 15c, and 15d, onto the conductive material, and finally, performing an etching process to remove the resist and leave only the resistance heating portion 15a and the first, second, and third electrode portions 15b, 15c, and 15d of the conductive layer 15.

The resistance heating member 13 is then placed inside the roller body 11 and secured with an adhesive or the like. The insulating layer is secured such that the conductive layer 15 is facing the axis of the roller body 11, and the first, second, and third electrode portions 15b, 15c, and 15d form continuous tracks around the inner circumference of the heat roller 10. In this embodiment, since the first, second, and third electrode portions 15b, 15c, and 15d are provided at the end portions of the resistance heating member 13, it is unnecessary to provide separate circular electrodes to contact the inner circumference of the heat roller 10.

In this case, referring again to FIG. 1, the heat roller 10 is provided with first, second, and third support electrodes 16, 18a and 18b. The first, second, and third support electrodes 16, 18a and 18b are provided with first, second, and third brushes 17, 19a and 19b at the tips thereof and are arranged such that the first, second, and third brushes 17, 19a, and 19b contact the first, second, and third electrode portions 15b, 15c, and 15d, respectively. As the heat roller 10 rotates the first, second, and third brushes 17, 19a, and 19b remain in sliding contact with the corresponding first, second, and third electrode portions 15b, 15c, and 15d.

As described above and as shown in FIG. 4, when the resistance heating member 13 is installed in the roller body 11, the top area 14b overlaps and covers the bottom area 14a. Thus, the insulating film 14 covers all of the inner circumference of the roller body 11 and the brushes 17, 19a and 19b do not contact the roller body 11 directly. Further, the bottom and top areas 14a and 14b are arranged to overlap in relation to the rotational direction (indicated by an arrow B in FIG. 4) of the heat roller 10 so that the brushes 17, 19a, 19b do not catch on the insulating film 14 and thereby, the life of the brushes 17, 19a, 19b and the insulating film 14 is extended.

Further, as shown in FIG. 3, the provision of the bottom area 14a allows the connector 15g to connect the third electrode portion 15d and the resistance heating portion 15a through the bottom area 14a (insulated by the top area 14b) to ensure that the first, second, and third brushes 17, 19a and 19b do not accidentally make contact with an incorrect part of the conductive layer 15.

The operation of the heat roller 10 is now described. FIG. 7 is a simplified schematic diagram of the heating member driving unit 50 and the heat roller 10. The heating member driving unit 50 includes a controller 51, a switch circuit 30, a voltage source V0, an input port I1, and three output ports O1, O2, O3. In a particular case, the controller 51 and the voltage source V0 may be shared elements with related elements in the image forming device.

The controller 51 receives data regarding paper size and the like from the image forming device (for example, from an operation panel (not shown) of the image forming device) via input port I1 and controls the switch circuit 30 according to the data received.

The voltage source V0 is electrically connected to the output port O1 and to the switch circuit 30. The switch circuit 30, as controlled by the controller 51, electrically connects the voltage source V0 to either the output port O2 or the output port O3. The output ports O1, O2, and O3 are connected to the first, second, and third support electrodes 16, 18a, and 18b, respectively.

Thus, the controller 51 controls the switch circuit 30 to apply a voltage from the voltage source V0 across the first support electrode 16 and a selected one of the second and third support electrodes 18a and 18b.

In particular, if the switch circuit 30 is set for the output O2, a voltage is applied across the first and second electrode portions 15b and 15c and heat is generated by both serpentine patterns 15e and 15f along the entire length of the heat roller 10. If the switch circuit 30 is set for the output O3, a voltage is applied across the first and third electrode portions 15b and 15d and only the first serpentine pattern 15e generates heat such that only a corresponding portion of the heat roller 10 is heated.

An alternative heating member driving unit 50A is shown in FIG. 8 as attached to the heat roller 10. The alternative heating member driving unit 50A is similar to the heating member driving unit 50 described above and identical elements are assigned the same reference numbers. The heat member driving unit 50A includes the input port I1, the output ports O1, O2, O3, a controller 51A, two switch circuits 31, 32, and four voltage sources V1, V2, V3, V4.

The switch circuit 31 is controlled by the controller 51A to connect the output port O2 to either of voltage source V1 or voltage source V2 and the switch circuit 32 is controlled by the controller 51A to connect the output port O1 to either of voltage source V3 or voltage source V4. The output port O3 and the voltage sources V1, V2, V3, V4 are connected to ground.

As above, the output ports O1, O2, and O3 are connected to the first, second, and third support electrodes 16, 18a, and 18b, respectively, and the controller 51A receives data through the input port I1.

Thus, the controller 51A controls the switch circuits 31, 32 to selectively apply a voltage from either voltage source V1 or V2 to the first support electrode 16, and selectively apply a voltage from either voltage source V3 or V4 to the second support electrode 18a. Thus, a voltage applied to each of the serpentine patterns 15e and 15f can be varied.

As an example, if the switch circuit 31 is set for the voltage source V1, a predetermined voltage is applied to the second serpentine pattern 15f and the second serpentine pattern generates a predetermined amount of heat, whereas if the switch circuit 31 is set for the voltage source V2, a different predetermined voltage is applied to the second serpentine pattern 15f and a different predetermined amount of heat is generated. The voltage applied to the first serpentine portion 15e is similarly controlled. Thus, corresponding portions of the heat roller 10 are heated according to the voltage applied to the first and second serpentine portions 15e and 15f.

As a further alternative of the first embodiment, an alternative resistance heating member 13' is shown in FIG. 5. The resistance heating member 13' is similar to the resistance heating member 13 described above except that the proportions of the first and second serpentine portions 15e' and 15f' of resistance heating portion 15a' on conductive layer 15' are arranged such that the first serpentine portion 15e' covers approximately three-quarters of the insulating layer 14 and the second serpentine portion 15f' covers the remaining part of the insulating layer 14. This arrangement may be useful in an image forming device in which an edge of the recording sheet P is generally aligned near an end of the heat roller 10. It should be noted that either of the heating member driving units 50, 50A described above may be used with the alternative resistance heating member 13'.

A resistive heating member 213 according to a second embodiment of the invention is now described with reference to FIG. 6. Note, elements in this embodiment that are identical to elements in the previous embodiment are assigned the same reference numerals and the description thereof is omitted.

As shown in FIG. 6, the insulating layer 14 is provided with a conductive layer 215 that includes the first, second, and third electrode portions 15b, 15c, and 15d of the first embodiment. In this embodiment, the conductive layer 215 is further provided with a fourth electrode portion 215i, positioned outside of the third electrode portion 15d, i.e., at the right side in FIG. 6. Also, rather than the resistance heating portion 15a of the previous embodiment, the conductive layer 215 includes a resistance heating portion 215a.

The resistance heating portion 215a includes first, second, and third resistance heating patterns, referred to as first, second, and third serpentine patterns 215e, 215f, and 215h, each arranged over a predetermined area along the X direction of the insulating layer 14 and over all of the Y direction of the insulating layer 14 except the bottom area 14a. In particular, along the X direction, the first serpentine pattern 215e is arranged on approximately a left quarter of the insulating layer 14, the second serpentine pattern 15f is arranged on approximately a right quarter of the insulating layer 14, and the third serpentine pattern 15h is arranged between the first and second serpentine patterns 15e and 15f, as shown in the view of FIG. 6. The first serpentine portion 215e includes first and second end portions 251a and 251b, the second serpentine portion 215f includes first and second end portions 252a and 252b, and, similarly, the third serpentine portion 215h includes first and second end portions 253a and 253b.

The second end portion 251b of the first serpentine pattern 215e and the first end portion 253a of the third serpentine pattern 215h are connected near the bottom area 14a. An electrical connector 215j is connected at the connection of the end portions 251b and 253a, runs along the bottom area 14a, and is connected to the third electrode portion 15d.

Similarly, the second end portion 252b of the second serpentine pattern 215f is connected with the first end portion 253b of the third serpentine pattern 215h near the bottom area 14a and an electrical connector 215g is connected at the connection of the end portions 252b and 253b, runs along the bottom area 14a, and is connected to the fourth electrode portion 15i.

Further, the first end portion 251a of the first serpentine pattern 215e is electrically connected to the first electrode portion 15b in the top area 14b and the first end portion 252a of the second serpentine pattern 215f is electrically connected to the second electrode portion 15c in the top area 14b.

Thus, if a voltage is applied across the first electrode portion 15b and the second electrode portion 15c, current flows through all of the first, second, and third serpentine patterns 215e, 215f, and 215h, since the first, second, and third serpentine portions 215e, 215f, and 215h are connected in series. If a voltage is applied across the first electrode portion 15b and the third electrode portion 15d, current flows in the first and third serpentine patterns 215e and 215h. If a voltage is applied across the second electrode portion 15c and the third electrode portion 15d, current flows only in the second serpentine pattern 215f. If a voltage is applied across the fourth electrode portion 15i and the third electrode portion 15d, current flows only in the third serpentine pattern 215h.

In this embodiment, a fourth support electrode and brush (not shown) are provided to contact the fourth electrode portion 15i. FIG. 9 shows an arrangement of a heating member driving unit 50B for this embodiment. The heating member driving unit 50B includes a controller 51B, a switch circuit 33, a voltage source V5, an input port I21, and four output ports O21, O22, O23, and O24. In a particular case, the controller 51B and the voltage source V5 may be shared elements with related elements in the image forming device.

The controller 51B receives data regarding recording sheet size and the like from the image forming device (for example, from an operation panel (not shown) of the image forming device) via input port 121 and controls the switch circuit 33 according to the data received.

The voltage source V5 is electrically connected to the output port O21 and to the switch circuit 33. The switch circuit 33 electrically connects the voltage source V5 to one of the output ports O22, O23, or O24. The output ports O21, O22, O23, and O24 are connected to the first, second, third support electrodes 16, 18a, 18b and the fourth support electrode (not shown), respectively.

Thus, the controller 51B controls the switch circuit 33 to selectively apply a voltage from the voltage source V5 across the first support electrode 16 and a selected one of the second, third, and fourth support electrodes 18a, 18b such that current flows through selected serpentine patterns 215e, 215f, 215h of the resistance heating portion 215a.

For example, if the switch circuit 33 is set for the output O22, a voltage is supplied across the first and second electrode portions 15b and 15c, current flows through the first, second, and third serpentine patterns 215e, 215f, and 215h, and heat is generated along the entire length of the heat roller 10. If the switch circuit 33 is set for the output O23, a voltage is supplied across the first and third electrode portions 15b and 15d, and only the first and third serpentine patterns 215e and 215h generate heat. Further, if the switch circuit 33 is set for the output O24, only the first serpentine pattern 15e generates heat and only a corresponding portion of the heat roller 10 is heated.

Note that by using alternative arrangements of the heating member driving unit 50B, similar to the alternative described above, voltages may be applied in other combinations or at varying levels to obtain a desired heating pattern.

Thus, with the arrangements described above, portions of the heat roller along the axis thereof may be selectively heated in order to provide a uniform temperature for fixing toner on the recording sheet while avoiding the problems of overheating or cooling. For example, even if a recording sheet having a small width is fed to the heat roller, the portion of the heat roller which does not contact the recording sheet can be prevented from overheating and an even distribution of heat may be maintained using an appropriate amount of power.

Although the structure and operation of a fixing unit and a heat roller for fixing unit are described herein with respect to the preferred embodiments, many modifications and changes can be made without departing from the spirit and scope of the invention.

For example, rather than using stainless steel for the conductive layer, another metal such as copper or iron may be used. In the case that copper foil is used, there is an advantage in that copper may be soldered easily, however, since copper has less resistance than stainless steel, the resistance heating portion 15a must be made thinner and longer than that for stainless steel. Further, since the copper oxidizes easier than stainless steel, if copper foil is used, it is preferable to cover the resistance heating portion 15a with a film made of polyimide resin or the like.

As another example modification, the conductive member 15 may be modified to include more than the described number of serpentine patterns or electrode portions. Further, the serpentine patterns may be formed having different widths. Further modifications may depend on the selection of appropriate materials and patterns depending on factors such as the type of image forming device, and the like.

Still further, the insulating sheet 14 may be secured on the roller body 11 by a method other than adhesion.

Still further, the voltage sources V0, V1, V2, V3, V4, V5, and switch circuits 30, 31, 32, 33 may be replaced by equivalent circuit elements. In particular, the voltage sources V0, V1, V2, V3, V4, V5 may be replaced by current sources. Further, the switch circuits 30, 31, 32, 33 may be controlled manually rather than by a controller.

The present disclosure relates to subject matter contained in Japanese Patent Application No. HEI 08-136349, filed on May 30, 1996, which is expressly incorporated herein by reference in its entirety.