WO2023075861A1

WO2023075861A1 - Heating element patterns for providing heating amount corresponding to various printing media

Info

Publication number: WO2023075861A1
Application number: PCT/US2022/029538
Authority: WO
Inventors: Sunhyung LEE; Hojin Ryu
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2021-10-27
Filing date: 2022-05-17
Publication date: 2023-05-04
Also published as: KR20230060140A

Abstract

An example fusing unit includes a flexible fusing belt, a backup member located outside the fusing belt to form a fusing nip with the fusing belt, and a heater substrate having a first surface including a heating element pattern and a second surface, opposite to the first surface, to heat the fusing belt at the fusing nip. The heating element pattern includes a first heating element having a first length, a second heating element having a second length that is greater than the first length, and a pair of third heating elements having a third length that is greater than the second length, and a resistance value of the second heating element is less than the resistance value of the first heating element.

Description

HEATING ELEMENT PATTERNS FOR PROVIDING HEATING AMOUNT CORRESPONDING TO VARIOUS PRINTING MEDIA

BACKGROUND

[0001] In a printer using an electrophotographic method, toner may be supplied to an electrostatic latent image formed on an image receptor to form a visible toner image on the image receptor, the toner image may be transferred to a print medium, and the transferred toner image may be fused on the print medium.

[0002] A fusing process may include a process of heating and pressing the toner. A fusing unit may include a heating member and a pressurization member, which are engaged with each other to form a fusing nip. The heating member may be heated by a heater. The print medium, to which the toner image is transferred, may be heated and pressed while passing through the fusing nip, and the toner image may be fused on the print medium.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] Various examples will be described below by referring to the following figures.

[0004] FIG. 1 is a view schematically illustrating a fusing unit according to an example.

[0005] FIG. 2 is a schematic cross-sectional view of a heater substrate shown in FIG. 1 according to an example.

[0006] FIG. 3 is a schematic plan view of the heater substrate shown in FIG. 1 according to an example.

[0007] FIG. 4 is a view for describing an operation of a plate-shaped heater according to an example.

[0008] FIG. 5 is a view illustrating a heater substrate according to a comparative example.

[0009] FIG. 6 is a plan view for describing a structure of heating element patterns in the heater substrate of FIG. 3 according to an example.

[0010] FIGS. 7 and 8 are schematic plan views of a heater substrate according to various examples.

[0011] FIG. 9 is a conceptual view illustrating an image forming apparatus including a fusing unit according to an example.

DETAILED DESCRIPTION

[0012] An electrophotographic printer may include a printing unit to form a visible toner image on a print medium P, for example, paper, and a fusing unit to fuse the toner image to the print medium P. The printing unit may include an exposure unit, a photosensitive drum, a developing device, a transfer unit, and the like. The exposure unit may irradiate light modulated according to image information on the surface of the photosensitive drum charged with a uniform surface electric potential to form an electrostatic latent image on the surface of the photosensitive drum. The developing device may supply toner to the electrostatic latent image formed on the photosensitive drum to develop the electrostatic latent image into a toner image. The transfer unit may transfer the toner image formed on the photosensitive drum to the print medium P. The toner image transferred to the print medium P may be maintained on the print medium P by an electrostatic force. The fusing unit may heat and press the toner image transferred to the print medium P to fuse the toner image on the print medium P. [0013] In order to increase printing speed and reduce energy consumption, a heated portion having a small amount of heat capacity may be included in the fusing unit. For example, a fusing belt having a thin film shape may be included as the heated portion. By including the fusing belt, the temperature of the fusing belt may rise to a fusing temperature, and printing may be performed within a quick time after power of a printer is turned on.

[0014] FIG. 1 is a view schematically illustrating a fusing unit 1 according to an example, FIG. 2 is a schematic cross-sectional view of a heater substrate 100 shown in FIG. 1 according to an example, and FIG. 3 is a schematic plan view of the heater substrate 100 shown in FIG. 1 according to an example. [0015] Referring to FIGS. 1 through 3, the fusing unit 1 may include a fusing belt 10, which is flexible, a backup member 30 that is located outside the fusing belt 10 to form a fusing nip 20 with the fusing belt 10, and the heater substrate 100. In an example, the heater substrate 100 has a first surface 101 including heating element patterns 105, and a second surface 102 that is an opposite surface to the first surface 101 to heat the fusing belt 10 in the fusing nip 20. A plate-shaped heater 2 may include the heater substrate 100 and the heating element patterns 105.

[0016] The heater substrate 100 may be located inside the fusing belt 10 to heat the fusing belt 10. The backup member 30 may be located outside the fusing belt 10 to face the heater substrate 100. A pressurization member 40 may press at least one of the heater substrate 100 and the backup member 30. By the pressing force of the pressurization member 40, the heater substrate 100 and the backup member 30 may press each other so that the fusing nip 20 may be formed. The heater substrate 100 may heat the fusing belt 10 in the fusing nip 20 so as to heat a print medium P having various widths. Based on the print medium P having a surface on which a toner image T is formed passing through the fusing nip 20, the toner image T may be fused on the print medium P by heat and pressure.

[0017] The fusing belt 10 may include a flexible base layer (not shown). The base layer may include a thin metal layer including stainless steel, nickel, nickel copper, or the like. The base layer may also include a polymer film, such as a polyimide film, a polyamide film, a polyimideamide film, or the like having heat resistance and wear resistance that may withstand a fusing temperature. A release layer (not shown) may be provided on a side surface or both sides of the backup member 30 of the base layer. The release layer may include a resin layer having isolation properties. The release layer may include perfluoroalkoxy (PFA), polytetrafluoroethylene (PTFE), fluorinated ethylene prophylene (FEP), or the like. In order to form a relatively wide and flat fusing nip 20, an elastic layer (not shown) may be located between the base layer and the release layer. The elastic layer (not shown) may include a material having a heat resistance to withstand the fusing temperature. For example, the elastic layer may include a rubber material such as fluorine rubber, silicone rubber, etc.

[0018] The backup member 30 may have a shape of a roller to drive the fusing belt 10 while being rotated by being pressed with respect to the heater substrate 100 with the fusing belt 10 therebetween. For example, the backup member 30 may include a core 31 that extends in a long side direction LD, and an elastic layer 32 on an outer periphery of the core 31 . The core 31 may include, for example, a metal shaft, a metal cylinder, or the like. In an example, the elastic layer 32 may include a material such as rubber, thermoplastic elastomer, or the like. A release layer (not shown) may be included on an outer surface of the elastic layer 32. The release layer may include perfluoroalkoxy (PFA), polytetrafluoroethylene (PTFE), fluorinated ethylene prophylene (FEP), or the like. [0019] The pressurization member 40 may provide, for example, a pressing force toward the backup member 30 to the heater substrate 100. In an example, the pressurization member 40 may provide a pressing force to a heater holder 50 on which the heater substrate 100 is supported, or to a pressurization bracket 60 connected to the heater holder 50. A structure for providing a pressing force to the heater substrate 100 is not limited to the example structure shown in FIG. 1.

[0020] Referring to FIGS. 2 and 3, the heater substrate 100 may include a thermal conductive substrate. For example, the heater substrate 100 may include a ceramic substrate. For example, alumina (AI2O3), aluminum nitride (AIN), or the like may be used as a ceramic material. The heater substrate 100 may have the first surface 101 and the second surface 102. The heating element patterns 105, a conductor pattern 140 to provide a conductive path, and electrodes 151 , 152, 153, and 154 to supply power may be located on the first surface 101 of the heater substrate 100.

[0021] An electric insulating layer 103 may be provided on the first surface 101 of the heater substrate 100. The electric insulating layer 103 may cover the heating element patterns 105, the conductor pattern 140, and the electrodes 151 , 152, 153, and 154. The electric insulating layer 103 may function as a protective layer to protect the heating element patterns 105, the conductor pattern 140, and the electrodes 151 , 152, 153, and 154. The electric insulating layer 103 may be, for example, a glass layer. The second surface 102 of the heater substrate 100 may face the fusing belt 10. The second surface 102 may produce friction with the driving fusing belt 10. In order to prevent abrasion of the heater substrate 100 or abrasion of the fusing belt 10, an abrasion prevention layer 104 may be provided on the second surface 102. The abrasion prevention layer 104 may include a material having a small frictional coefficient. The abrasion prevention layer 104 may be, for example, a glass layer.

[0022] The heating element patterns 105 may be heated by receiving electricity through the electrodes 151 , 152, 153, and 154, and the conductor pattern 140. The heating element patterns 105 may include a plurality of heating elements arranged to be apart from each other in a short side direction of the heater substrate 100. The heating element patterns 105 may include a metal heating material, for example, a silver (Ag)-palladium (Pd) alloy, or the like. The heater substrate 100 may be heated by heating of the heating element patterns 105, and the temperature of the heater substrate 100 may reach a fusing temperature, for example, 80-150°C.

[0023] Referring to FIG. 3, the heating element patterns 105 may include heating elements having different lengths. For example, the heating element patterns 105 may include a first heating element 110 having a first length L1 , a second heating element 120 having a second length L2 that is greater than the first length L1 , and a pair of third heating elements 130 having a third length L3 that is greater than the second length L2. Here, the length is defined as a length in the long side direction LD of the heater substrate 100.

[0024] FIG. 4 is a view for describing an operation of a plate-shaped heater 2 according to an example. Referring to FIG. 4, a controller 200 may selectively control driving of the first heating element 110, the second heating element 120, and the third heating elements 130 according to the type of print media. For example, for heating of a first print medium P1 having a smallest width, the controller 200 may drive the first heating element 110 that is the shortest. For heating of a second print medium P2 having a greater width than the first print medium P1 , the controller 200 may drive the second heating element 120 that is longer than the first heating element 110. For heating of a third print medium P3 having a greater width than the second print medium P2, the controller 200 may drive the third heating elements 130 that are longer than the second heating element 120. As described above, by selectively controlling driving of heating elements having different lengths, a phenomenon in which some areas are overheated during the heating process for the other print media having different widths, may be reduced. The first print medium P1 may be a print medium having an A5 size, the second print medium P2 may be a print medium having a B5 size, and the third print medium P3 may be a print medium having a A4 size. However, examples are not limited thereto.

[0025] As the lengths L1 , L2, and L3 of the first, second, and third heating elements 110, 120, and 130 in the heating element patterns 105 are different, the amount of heat generated by each of the first, second, and third heating elements 110, 120, and 130 may be different. For example, based on the first, second, and third heating elements 110, 120 and 130 having the same widths according to the short side direction SD and having different lengths according to the long side direction LD, the amount of heat generated based on the same voltage being applied to the first, second, and third heating elements 110, 120, and 130, may be different.

[0026] FIG. 5 is a view illustrating a heater substrate 1000 according to a comparative example. Referring to FIG. 5, in the heater substrate 1000 according to the comparative example, first, second, and third heating elements 1101 , 1201 , and 1301 may be different in length, but the remaining conditions, such as materials, widths, and thicknesses, may be the same.

[0027] The amount of heat generated by the heating element may be inversely proportional to the resistance value of the heating element. The resistance value of the heating element may be proportional to the length of the heating element. Accordingly, the resistance value of the first heating element 1101 having the shortest length may be the smallest, and the amount of heat generated by the first heating element 1101 may be the greatest. If the first, second, and third heating elements are arranged in the order of the amount of heat generation, the first heating element 1101 , the second heating element 1201 , and the third heating element 1301 may be in this order. [0028] On the other hand, in order to fuse the toner to the print media P1 , P2, and P3, the heat capacity of the print medium may vary depending on the size of the print media P1 , P2, and P3. For example, the heat capacity of the third print medium P3 having the largest width may be the greatest, and the heat capacity of the first print medium P1 having the smallest width may be the smallest. If the print media P1 , P2, and P3 are arranged in the order of the amount of heat capacity, the third print medium P3, the second print medium P2, and the first print medium P1 may be in this order.

[0029] As with the heater substrate 1000 according to the above comparative example, based on the lengths of the first through third heating elements 1101 , 1201 and 1301 in the heating element patterns 105 being designed to correspond to the widths of the print media P1 , P2, and P3, the amount of heat generated by the first through third heating elements 1101 , 1201 and 1301 and the heat capacity of the print media P1 , P2, and P3 may be opposed to each other. Thus, the amount of heat generated by the first heating element 1101 is greater than the heat capacity of the first print medium P1 and unnecessary energy is consumed, while the heat generated by the third heating element 1301 is less than the heat capacity of the third print medium P3, which may cause a phenomenon in which image quality is lowered.

[0030] FIG. 6 is a plan view for describing a structure of the heating element patterns 105 in the heater substrate 100 according to an example. FIGS. 7 and 8 are schematic plan views of a heater substrate according to various examples. Referring to FIGS. 2 and 6, in the heating element patterns 105, as the length of the first through third heating elements 110, 120, and 130 increases, the amount of heat may increase. As an example, the heating element patterns 105 may use a pair of third heating elements 130. In that case, the resistance value of the pair of third heating elements 130 may be less than the resistance value of the second heating element 120 so that the amount of heat generated by the third heating element 130 may be greater than the amount of heat generated by the second heating element 120, and the resistance value of the second heating element 120 may be less than the resistance value of the first heating element 110 so that the amount of heat generated by the second heating element 120 may be greater than the amount of heat generated by the first heating element 110.

[0031] The pair of third heating elements 130 may be arranged on both ends of the heater substrate 100 in the short side direction SD, and the first heating element 110 and the second heating element 120 may be arranged between the pair of third heating elements 130. The first heating element 110, the second heating element 120, and the pair of third heating elements 130 may be arranged to be apart from each other in the short side direction SD. The first heating element 110, the second heating element 120, and the third heating elements 130 may be arranged symmetrically with respect to the center of the long side direction LD. The material of the first heating element 110, the second heating element 120, and the pair of third heating elements 130 may be the same. [0032] The pair of third heating elements 130 may be connected to each other in parallel. Thus, the total resistance value of the pair of third heating elements 130 may be less than the resistance value of the third heating element 130 so that the amount of heat generated by the pair of third heating elements 130 may be increased. Thus, the amount of heat generated by the pair of third heating elements 130 may be greater than the amount of heat generated by the first heating element 110 and greater than the amount of heat generated by the second heating element 120.

[0033] One end of the pair of third heating elements 130 may be connected to the first electrode 151 , and the other end thereof may be connected to the second electrode 152 so that the pair of third heating elements 130 may be connected to each other in parallel. The conductor pattern 140 may be disposed between one end of the first heating element 110 and the first electrode 151 and between the other end of the third heating element 130 and the second electrode 152.

[0034] One end of each of the first heating element 110 and the second heating element 120 may be connected to the third electrode 153. The other end of the second heating element 120 may be connected to the second electrode 152, and the other end of the first heating element 110 may be connected to the fourth electrode 154. The third electrode 153 and the fourth electrode 154 may be arranged between the first electrode 151 and the second electrode 152.

[0035] A width H2 of the second heating element 120 according to the short side direction SD may be greater than a width H1 of the first heating element 110 according to the short side direction SD so that the amount of heat generated by the second heating element 120 may be greater than the amount of heat generated by the first heating element 110. The width H2 of the second heating element 120 may be 1 .2 to 3 times the width H1 of the first heating element 110. By increasing the width H2 of the second heating element 120, the resistance value of the second heating element 120 may be adjusted to be less than the resistance value of the first heating element 110. Thus, the amount of heat generated by the second heating element 120 may be greater than the amount of heat generated by the first heating element 110. The width H2 of the second heating element 120 according to the short side direction SD may be greater than the width H3 of the third heating element 130 according to the short side direction SD. The width H2 of the second heating element 120 may be 1 .2 to 3 times the width H3 of the third heating element 130.

[0036] The width H 1 of the first heating element 110 according to the short side direction SD may be different from the width H3 of each of the pair of third heating elements 130 according to the short side direction SD. For example, the width H1 of the first heating element 110 may be greater than the width H3 of each of the pair of third heating elements 130. In another example, as shown in FIG. 7, the width H1 of a first heating element 110A may be less than the width H3 of each of the pair of third heating elements 130. However, the width H1 of the first heating elements 110 and 110A is not limited thereto, and as shown in FIG. 8, the width H1 of a first heating element 110B may be the same as the width H3 of the third heating element 130.

[0037] As described above, in the fusing unit 1 according to an example, the heating element patterns 105 may include a first heating element 110, a second heating element 120, and a pair of third heating elements 130, and the width H2 of the second heating element 120 may be greater than the width H1 of the first heating element 110 so that the amount of heat corresponding to various print media P1 , P2, and P3 may be provided. In other words, in order to heat the third print medium P3 having the largest width, a pair of third heating elements 130 may generate the largest amount of heat, in order to heat the second print medium P2 having the second largest width, the second heating element 120 having a width H2 according to the short side direction SD may generate the second largest amount of heat, and in order to heat the first print medium P1 having the smallest width, the first heating element 110 may generate the smallest amount of heat. It is therefore possible to secure good image quality while preventing unnecessary energy consumption in an image forming process for various sizes of print media.

[0038] In the above-described example, the width H2 of the second heating element 120 is greater than the width H1 of the first heating element 110 so that the amount of heat generated by the second heating element 120 is greater than the amount of heat generated by the first heating element 110. However, examples are not limited thereto. For example, in order to make the amount of heat generated by the second heating element 120 be greater than the amount of heat generated by the first heating element 110, the thickness of the second heating element 120 may be greater than the thickness of the first heating element 110.

[0039] FIG. 9 is a conceptual view illustrating an image forming apparatus 300 including a fusing unit 1 according to an example. Referring to FIG. 9, the image forming apparatus 300 may include an image forming unit 330 to transfer a toner image to the print medium P, and the fusing unit 1 according to the abovedescribed examples to fuse the toner image on the print medium P by heating and pressing the print medium P to which the toner image is transferred. The image forming apparatus 300 may supply the print medium P to the image forming unit 330, and may include a paper feeder 310, and a paper discharging unit 320 in which the print medium P, on which the toner image is fused, is loaded. A printing path 302 may connect the paper feeder 310 to the paper discharging unit 320. The print medium P may include a first print medium P1 , a second print medium P2 (see FIG. 4) having a width greater than the first print medium P1 (see FIG. 4), and a third print medium P3 (see FIG. 4) having a width greater than the second print medium P2 (see FIG. 4). [0040] The print medium P loaded in the paper feeder 310 may be drawn out one by one and may be conveyed along the printing path 302. To this end, a pickup roller 312 may draw one sheet of the print medium P from a paper feeding tray 311. Transporting rollers 313 may transport the drawn print medium P along the printing path 302.

[0041] In an example, the paper feeder 310 has a shape of a paper feeding cassette. However, an example of the paper feeder 310 is not limited thereto.

[0042] The image forming unit 330 may transfer the toner image to the print medium P transported along the printing path 302 by using an electrophotographic method. The image forming unit 330 may include a developing device 340, an exposure unit 350, and a transfer unit 370.

[0043] The image forming unit 330 according to an example may selectively print a monochrome image or a color image on the print medium P.

[0044] For color printing, the developing device 340 may include, for example, four developing devices 340 for developing images of cyan (C), magenta (M), yellow (Y), and black (K) colors, respectively. Developing agents of cyan (C), magenta (M), yellow (Y), and black (K) colors, for example, toners may be accommodated in four developing devices 340, respectively. Cyan (C), magenta (M), yellow (Y), and black (K) color toners may be accommodated in four toner supply containers 345, respectively, and cyan (C), magenta (M), yellow (Y), and black (K) color toners may also be supplied to four developing devices 340 from the four toner supply containers 345, respectively. The image forming apparatus 300 may further include a developing device to accommodate and develop toner of various colors such as light magenta, white, and the like, in addition to the above-described colors. Based on the accommodated toner being exhausted, the respective toner supply container 345 may be replaced. The developing device 340 may be detached from the image forming apparatus 300 through a door (not shown).

[0045] Hereinafter, an example image forming unit 330 having four developing devices 340 will be described. Based on C, M, Y and K being attached to reference numerals unless otherwise noted, it may refer to a component to develop images of C, M, Y, and K colors. [0046] The developing device 340 may supply the toner accommodated therein to the electrostatic latent image formed on a photosensitive drum 341 .

[0047] The photosensitive drum 341 is an example of a photoconductor having a surface on which an electrostatic latent image may be formed. The photosensitive drum 341 may include a conductive metal pipe and a photosensitive layer formed on the outer periphery of the conductive metal pipe. A charging roller 342 may charge the surface of the photosensitive drum 341 to a uniform electric potential.

[0048] The exposure unit 350 may irradiate light modulated corresponding to the image information on the photosensitive drum 341 to form an electrostatic latent image on the photosensitive drum 341. A light emitting diode (LED) exposure unit that uses an LED as a light source, or a laser scanning unit (LSU) that uses a laser diode as a light source may be included as the exposure unit 350.

[0049] A developing roller 343 may supply the developing agent accommodated in the developing device 340, for example, the toner, to the photosensitive drum 341 to develop the electrostatic latent image into a visible toner image. A developing bias voltage may be applied to the developing roller 343. Based on a one-component developing method being used, the toner may be accommodated in the toner supply container 345 of the developing device 340. Based on a two-component developing method being used, toner or toner and a carrier may be accommodated in the toner supply container 345 of the developing device 340. Although not shown, the developing device 340 may further include a supply roller to supply the developing agent accommodated in the toner supply container 345 to the developing roller 343, a regulating member that is attached to the surface of the developing roller 343 to regulate the amount of the developing agent supplied to a developing area in which the photosensitive drum 341 and the developing roller 343 face each other, and an agitator to agitate the developing agent accommodated in the toner supply container.

[0050] The transfer unit 370 may include an intermediate transfer belt 371 , an intermediate transfer roller 372, and a transfer roller 373. The toner image developed on the photosensitive drum 341 of each of developing devices 340C, 340M, 340Y, and 340K may be intermittently transferred to the intermediate transfer belt 371 . The intermediate transfer belt 371 may be supported by support rollers 374 and 375 and circulated.

[0051] The intermediate transfer belt 371 may be a member on which the toner image is formed on the surface of the intermediate transfer belt 371 , and the surface of the intermediate transfer belt 371 , on which the toner image is formed, is movable toward the transfer roller 373. The intermediate transfer belt 371 may function as an image transporting member to transport the toner image. [0052] Four intermediate transfer rollers 372 may be arranged in a position to face the photosensitive drum 341 of the developing devices 340C, 340M, 340Y and 340K, with the intermediate transfer belt 371 therebetween. An intermediate bias voltage for intermediately transferring the toner image developed on the photosensitive drum 341 to the intermediate transfer belt 371 may be applied to the four intermediate transfer rollers 372. Instead of the intermediate transfer roller 372, a corona transfer unit or a pin scorotron-type transfer unit may also be included. The transfer roller 373 may be located to face the intermediate transfer belt 371. A transfer bias voltage for transferring the toner image intermediately- transferred to the intermediate transfer belt 371 to the print medium P may be applied to the transfer roller 373.

[0053] Toner images overlappingly transferred onto the intermediate transfer belt 371 may be transferred to the print medium P by the transfer bias voltage applied to the transfer roller 373.

[0054] The fusing unit 1 may fuse the toner image on the print medium P by heating and pressing the print medium P to which the toner image is transferred.

[0055] It should be understood that examples described herein should be considered in a descriptive sense and not for purposes of limitation. Descriptions of features or aspects within each example should typically be considered as available for other similar features or aspects in other examples. While examples have been described with reference to the figures, it will be understood that various changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims.

Claims

WHAT IS CLAIMED IS:

1 . A fusing unit comprising: a flexible fusing belt; a backup member located outside the fusing belt to form a fusing nip with the fusing belt; and a heater substrate having a first surface including a heating element pattern, and a second surface, opposite to the first surface, to heat the fusing belt at the fusing nip, wherein the heating element pattern comprises a first heating element having a first length, a second heating element having a second length that is greater than the first length, and a pair of third heating elements having a third length that is greater than the second length, and wherein a resistance value of the second heating element is less than a resistance value of the first heating element.

2. The fusing unit of claim 1 , wherein a width of the second heating element according to a short side direction of the second surface is greater than a width of the first heating element according to the short side direction.

3. The fusing unit of claim 1 , wherein the pair of third heating elements are arranged on both ends of the heater substrate in a short side direction of the second surface, and the first heating element and the second heating element are arranged between the pair of third heating elements.

4. The fusing unit of claim 1 , wherein an amount of heat generated by the second heating element is greater than an amount of heat generated by the first heating element, and wherein an amount of heat generated by the second heating element is less than an amount of heat generated by the pair of third heating elements.

5. The fusing unit of claim 4, wherein a width of the second heating element according to a short side direction of the second surface is greater than a width of each of the pair of third heating elements according to the short side direction.

6. The fusing unit of claim 5, wherein a width of the first heating element according to the short side direction is different from a width of each of the pair of third heating elements according to the short side direction.

7. The fusing unit of claim 1 , wherein the first heating element, the second heating element, and the pair of third heating elements comprise a same material.

8. The fusing unit of claim 1 , wherein one end of the pair of third heating elements is connected to a first electrode, and the other end of the pair of third heating elements is connected to a second electrode so that the pair of third heating elements are connected in parallel.

9. The fusing unit of claim 8, wherein one end of the first heating element and one end the second heating element are connected to a third electrode, the other end of the second heating element is connected to the second electrode, and the other end of the first heating element is connected to a fourth electrode.

10. The fusing unit of claim 9, wherein the third electrode and the fourth electrode are arranged between the first electrode and the second electrode.

11. An image forming apparatus comprising: an image forming unit to transfer a toner image to a print medium; and a fusing unit to fuse the toner image on the print medium by heating and pressing the print medium to which the toner image is transferred, wherein the fusing unit comprises: a flexible fusing belt; 16 a backup member located outside the fusing belt to form a fusing nip with the fusing belt; and a heater substrate comprising a first surface including a heating element pattern, and a second surface, opposite to the first surface, to heat the fusing belt at the fusing nip; wherein the heating element pattern comprises a first heating element having a first length, a second heating element having a second length that is greater than the first length, and a pair of third heating elements having a third length that is greater than the second length, and wherein a resistance value of the second heating element is less than a resistance value of the first heating element.

12. The image forming apparatus of claim 11 , wherein a width of the second heating element according to a short side direction of the second surface is greater than a width of the first heating element according to the short side direction.

13. The image forming apparatus of claim 12, wherein the pair of third heating elements are arranged on both ends of the heater substrate in a short side direction of the second surface, and the first heating element and the second heating element are arranged between the pair of third heating elements.

14. The image forming apparatus of claim 11 , wherein an amount of heat generated by the second heating element is greater than an amount of heat generated by the first heating element, and wherein an amount of heat generated by the second heating element is less than an amount of heat generated by the pair of third heating elements.

15. The image forming apparatus of claim 14, wherein a width of the second heating element according to a short side direction of the second surface is greater than a width of each of the pair of third heating elements according to the short side direction.