Image forming apparatus including fusing device

A fusing device includes a first roller, a second roller forming a fusing nip part together with the first roller, an adjustment mechanism changing posture of the first roller between first and second postures, a separation member, a turning mechanism, an abutting member, and an isolation mechanism. The turning mechanism turns the separation member so that the separation member approaches the first roller when the posture of the first roller is changed from the second posture to the first posture. The abutting member abuts on the first roller to form a gap between the separation member and the first roller when the first roller is in the first posture. The isolation mechanism separates the abutting member from the first roller in a process when posture of the first roller is changed from the first posture to the second posture.

INCORPORATION BY REFERENCE

The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2013-077704, filed Apr. 3, 2013. The contents of this application are incorporated herein by reference in their entirety.

BACKGROUND

The present disclosure relates to a fusing device to fuse a toner image to a sheet and an image forming apparatus to which the fusing device is applied.

Fusing devices for image forming apparatuses include a fusing nip part formed in a manner that a fusing roller is in press contact with a pressure roller. Pressure and heat are applied to a sheet when the sheet passes through the fusing nip part, thereby fusing to the sheet a toner image transferred to the sheet. Incidentally, toner melted after the sheet passes through the fusing nip part may form an adhesive layer. This may cause the sheet to adhere to the peripheral surface of the fusing roller. In this case, the sheet may wind around the fusing roller. In order to prevent such winding, a separation member is provided downstream of the fusing nip part in the rotation direction of the fusing roller. The separation member is arranged close to the fusing roller to separate the sheet, which starts winding to the fusing roller, from the fusing roller.

The separation member includes a plate-shaped member extending in the axial direction of the fusing roller. Where the tip end of the plate-shaped member is always in contact with the peripheral surface of the fusing roller, the peripheral surface of the fusing roller may be scared. If a toner image is fused to a sheet with the use of the fusing roller of which peripheral surface is scared, the scar may appear on the toner image. This scar may cause an image defect. For this reason, a gap is left between the tip end of the plate-shaped member and the peripheral surface of the fusing roller. This gap is formed with the use of an abutting member mounted on the end of the separation member. The known abutting member is always in contact with the peripheral surface of the fusing roller in a non-sheet passing region of the fusing roller.

SUMMARY

A fusing device according to the first mode of the present disclosure includes a first roller, a second roller, an adjustment mechanism, a separation member, a turning mechanism, an abutting member, and an isolation mechanism. The first roller rotates about its axis. The second roller rotates about is axis. The second roller forms a fusing nip part together with the first roller. The adjustment mechanism changes posture of the first roller or the second roller between first posture and second posture to adjust a nip pressure of the fusing nip part. The adjustment mechanism sets the first or second roller in the first posture to allow the first or second roller to be in press contact with the second or first roller by a first pressure. The adjustment mechanism sets the first or second roller in the second posture to allow the first or second roller to be in press contact with the second or first roller by a second pressure which is reduced from the first pressure. The separation member is arranged downstream of the fusing nip part in a direction of rotation of the first roller. The separation member includes a plate-shaped member extending in an axial direction of the first roller. The turning mechanism turns the separation member about an axis parallel to the axial direction of the first roller. The turning mechanism turns the separation member so that a tip end of the plate-shaped member approaches a peripheral surface of the first roller when the posture of the first or second roller is changed from the second posture to the first posture. The abutting member is mounted on each opposite end of the plate-shaped member of the separation member. The abutting member abuts, when the first or second roller is in the first posture, on the peripheral surface of the first roller in a vicinity of each opposite end of the first roller to form a gap between the tip end of the plate-shaped member and the peripheral surface of the first roller. The isolation mechanism allows the abutting member to abut on the peripheral surface of the first roller when the first or second roller is in the first posture and separates the abutting member from the peripheral surface of the first roller in the process when the posture of the first or second roller is changed from the first posture to the second posture.

A fusing device according to the second mode of the present disclosure further includes a heater of induction heating type in the fusing device according to the first mode. The heater heats the first roller. The first roller and the second roller rotate when the first or second roller is in the first posture. Further, the first and second rollers rotate when the first or second roller is in the second posture.

An image forming apparatus according to the third mode of the present disclosure includes an image forming section and a fusing device according to the first mode. The image forming section transfers a toner image to a sheet. The fusing device fuses the toner image to the sheet.

An image forming apparatus according to the fourth mode of the present disclosure includes an image forming section, a fusing device according to the second mode, a drive section, a mode switching section, and a controller. The image forming section transfers a toner image to a sheet. The fusing device fuses the toner image to the sheet. The drive section drives the first or second roller. The mode switching section switches an operating mode of the image forming apparatus between a first mode and a second mode, the first mode being a mode in which image formation is performed on a sheet to cause the sheet to pass through the fusing nip part, and the second mode being a standby state for image formation to cause no sheet to pass through the fusing nip part. The controller controls the adjustment mechanism so that the first or second roller is in the first posture in the first mode and so that the first or second roller is in the second posture in the second mode. Further, the controller controls the drive section so that the first and second rollers rotate in the first mode. Furthermore, the controller controls the drive section so that the first and second rollers rotate in the second mode.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be described below with reference to the accompanying drawings.FIG. 1is a cross sectional view showing an internal configuration of an image forming apparatus1according to one embodiment of the present disclosure. The image forming apparatus1in the present embodiment is a monochrome printer having a copying function. It is noted that the image forming apparatus according to the present disclosure is not limited to the monochrome printer having a copying function.

The image forming apparatus1includes a main body as a housing substantially in a rectangular parallelepiped shape. The main body is a casing to accommodate various devices for performing image formation on a sheet. The main body includes a body housing11substantially in a rectangular parallelepiped shape, a scanner housing12substantially in a rectangular parallelepipedal shape, and a joint housing13. The scanner housing12is arranged above the body housing11. The joint housing13joins the body housing11and the scanner housing12together.

An image forming section20for transfer of a toner image to a sheet is accommodated in the body housing11. A fusing device30to fuse a toner image to a sheet is accommodated in the joint housing13. A scanner unit17is accommodated in the scanner housing12. The scanner unit17optically reads an image on a document sheet to generate image data. The main body includes an in-body discharge section14. The in-body discharge section14is an in-body space surrounded by the upper surface of body housing11, the lower surface of the scanner housing12, and the left surface of the joint housing13. A sheet subjected to image formation is ejected to the in-body discharge section14.

The joint housing13is arranged on the right side surface of the main body10. In the joint housing13, a first sheet exit port154and a second sheet exit port155arranged above the first sheet exit port154are formed. The first sheet exit port154opens toward the in-body space for ejection of a sheet toward the in-body discharge section14. The second sheet exit port155also opens toward the in-body space similarly to the first sheet exit port154. The bottom of the in-body space is defined by an in-body discharge tray141. The in-body discharge tray141is located below the first sheet exit port154and forms the upper surface of the body housing11. The in-body discharge tray141receives a sheet ejected from the first sheet exit port154. A sub exit tray142is provided above the in-body discharge tray141. The sub exit tray142is fitted between the first sheet exit port154and the second sheet exit port155. The sub exit tray142receives a sheet ejected from the second sheet exit port155. Further, a sheet to be subjected to duplex printing is temporarily ejected to the sub exit tray142for switchback conveyance.

A sheet feed cassette15is detachably fitted in the lower part of the main body. The sheet feed cassette15accommodates a sheet to be subjected to image formation. Further, a manual feed tray16is provided on the right side surface of the main body. The manual feed tray16is used for manual sheet feed.

The image forming section20includes a photosensitive drum21. The image forming section20further includes an electrostatic charger22, an exposure device23, a developing device24, a transfer roller26, a cleaner27, and a static eliminator28, which are arranged around the photosensitive drum21. The photosensitive drum21rotates about its axis. Further, the photosensitive drum21has a peripheral surface on which an electrostatic latent image and a toner image are to be formed. The electrostatic charger22uniformly charges the peripheral surface of the photosensitive drum21. The exposure device23irradiates laser light to the peripheral surface of the photosensitive drum21to form an electrostatic latent image. The developing device24includes a development roller24A. The development roller24A supplies toner to the peripheral surface of the photosensitive drum21to develop an electrostatic latent image formed on the peripheral surface of the photosensitive drum21. The developing device24is replenished with toner supplied from a toner container25. The transfer roller26forms a transfer nip part together with the photosensitive drum21, thereby transferring a toner image to a sheet from the photosensitive drum21. The cleaner27cleans the peripheral surface of the photosensitive drum21after transfer of a toner image. The static eliminator28irradiates static eliminating light to the peripheral surface of the photosensitive drum21after transfer of a toner image to eliminate static electricity on the peripheral surface of the photosensitive drum21.

The fusing device30includes a fusing roller31(first roller) in which a heat source is provided and a pressure roller32(second roller). The pressure roller32forms a fusing nip part N together with the fusing roller31. The fusing device30performs fusing to melt toner to a sheet. Specifically, in the fusing nip part N, the fusing device30applies heat and pressure to a sheet to which a toner image is transferred in the transfer nip part. Thus, the toner image is fused to the sheet. A sheet subjected to fusing is ejected from the first sheet exit port154or the second sheet exit port155to the in-body discharge section14. The fusing device30will be described later in detail.

The scanner unit17in the scanner housing12includes a carriage (not shown) on which an image sensor, a light source, a mirror, etc. (each not shown) are boarded. The light source irradiates light to a document sheet. The mirror guides the reflected light from a document sheet to the image sensor. A contact glass, on which a document sheet is to be loaded, is fitted on the upper surface of the scanner unit17. Further, the main body includes a document cover18capable of covering the upper surface of the scanner unit17. The document cover18can press a document sheet loaded on the contact glass.

An operating panel19is mounted on the front surface of the scanner housing12. The operating panel19includes a LCD touch panel, a numeric keypad, etc. to receive input from the user. Through the operating panel19, various operating information to the image forming apparatus1is input.

A sheet conveyance path for conveyance of a sheet is formed in the interior of the main body. The sheet conveyance path includes a main conveyance path P1vertically extending from a part around the lower part to a part around the upper part of the main body. The main conveyance path P1goes through the image forming section20and the fusing device30. The downstream end of the main conveyance path P1is connected to the first and second sheet exit ports154and155. Further, a reversed sheet conveyance path P2extends from the most downstream end to a part around the upstream end of the main conveyance path P1. The reversed sheet conveyance path P2conveys a sheet from the most downstream end to the part around the upstream end of the main conveyance path P1in duplex printing.

The sheet feed cassette15includes a sheet accommodating section to accommodate a sheaf of sheets S. A pickup roller151and a sheet feed roller pair152are provided around the upper right part of the sheet feed cassette15. The pickup roller151picks up the uppermost sheet S of the sheet sheaf on a sheet-by-sheet basis. The sheet feed roller pair152sends out the picked sheet to the upstream end of the main conveyance path P1. A sheet loaded on the manual feed tray16is also sent out to the upstream end of the main conveyance path P1. A registration roller pair153is arranged upstream of the image forming section20in the main conveyance path P1. The registration roller pair153sends out a sheet to the transfer nip part with preset timing.

In order to perform simplex printing (image formation) on a sheet S, the sheet S is sent out from the sheet feed cassette15or the manual feed tray16to the main conveyance path P1, and a toner image is transferred to the sheet S in the image forming section20. Thereafter, the transferred toner is fused to the sheet S in the fusing device30. Then, the sheet S is ejected from the first sheet exit port154onto the in-body exit tray141. By contrast, in order to perform duplex printing on a sheet S, after transfer and fusing are performed on one surface of the sheet S, a part of the sheet S is ejected onto the sub exit tray142from the second sheet exit port155. Then, the sheet S is subjected to switchback conveyance to be returned to the part around the upstream end of the main conveyance path P1via the reverse sheet conveyance path P2. Thereafter, the other surface of the sheet S is subjected to transfer and fusing. Then, the sheet S is ejected from the first sheet exit port154onto the in-body exit tray141.

The configuration of the fusing device30will be described in detail below.FIG. 2is a schematic cross sectional view of the fusing device30.FIG. 2also shows a control blocks for the fusing device30. The fusing device30includes the fixing roller31and the pressure roller32. The fusing device30further includes a heater33of induction heating type and a fuser housing300. The heater33heats the fusing roller31. The fuser housing300accommodates the fusing roller31, the pressure roller32, the heater33, etc. An inlet301to receive a sheet (not shown) from the image forming section20is formed in the fuser housing300. Further, an outlet302to send out a sheet subjected to fusing is formed in the fuser housing300. A guide plate303to guide a sheet toward the fusing nip part N is provided at the inlet301.

The fusing roller31has a rotational axis31S (first rotational axis) extending back and forth (perpendicularly to the sheet ofFIG. 2). The pressure roller32has a rotational axis32S (second rotational axis) extending back and forth in parallel to the rotational axis31S. The fusing roller31and the pressure roller32are held in the fuser housing300(movable framework37and fixed framework36) so as to be respectively rotatable about the rotational axes31S and32S. In the present embodiment, rotational drive force to rotate the pressure roller32in the clockwise direction is applied to the pressure roller32. Accompanied by rotation of the pressure roller32, the fusing roller31follows and rotates in the anticlockwise direction.

The fusing roller31includes a fusing belt31A and an elastic roller member31B. The fusing belt31A is fitted around the roller member31B with clearance left. One example of the roller member31B may be a roller having a configuration in which an elastic layer is formed around a metal cored bar serving as a core. The metal cored bar may be made from SUS, or the like. The material for the elastic layer may be a silicon sponge, or the like. One example of the fusing belt31A may be an endless belt having a multilayered structure including a magnetic metal base material, an elastic layer, and a mold release layer. The magnetic metal base material is capable of being induction heated. Nickel or the like may be used as a material for the magnetic metal base material. The material for the elastic layer may be silicon rubber or the like. The material for the mold release layer may be fluorine based resin or the like. The pressure roller32has a rigidity higher than the fusing roller31. One example of the pressure roller32may be a roller including a non-magnetic metal cored bar serving as a core, an elastic layer, and a mold release layer. Aluminum or the like may be used as a material for the non-magnetic metal cored bar. The elastic layer is formed around the non-magnetic metal cored bar. The material for the elastic layer may be silicon rubber or the like. The mold release layer may be formed as an outermost layer of the pressure roller32.

The fusing roller31is in press contact with the pressure roller32. This deforms a part of the peripheral surface of the fusing roller31(fusing belt31A), which is in contact with the pressure roller32, into a concave arc shape. This contact part serves as the fusing nip part N. A sheet to be subjected to fusing enters the fuser housing300from the inlet301and is nipped in the fusing nip part N. The nipped sheet is conveyed by rotation of the fusing roller31and the pressure roller32about the respective rotational axes315and32S. The sheet is heated and pressed when passing through the fusing nip part N. Thereafter, the sheet is conveyed out from the outlet302. A separation member40is provided downstream of the fusing nip part N in the rotation direction of the fusing roller31. The separation member40is a member to prevent a sheet having passed through the fusing nip part N from winding around the peripheral surface of the fusing roller31.

The heater33includes a bobbin331and a coil332. The bobbin331has a curved shape along the outer peripheral surface of the fusing roller31and is arranged to face the fusing roller31. The coil332is wound around the bobbin331. High-frequency voltage for induction heating is applied to the coil332. When the high-frequency voltage is applied to the coil332, a magnetic path that passes through the fusing belt31A is formed. Accordingly, an eddy current flows in the magnetic metal base material of the fusing belt31A to heat the fusing belt31A.

For the fusing device30, a drive motor71(drive section), a nip pressure adjusting motor72(part of adjustment mechanism), and a controller73are provided additionally. The drive motor71generates rotational drive power to rotate the pressure roller32and the fusing roller31. The nip pressure adjusting motor72generates drive power to move the fusing roller31toward the pressure roller32with the rotational axes31S and32S kept in parallel to each other. The nip pressure adjusting motor72adjusts the nip pressure of the fusing nip part N.

When image formation is actually performed on a sheet (image formation mode), that is, when a sheet passes through the fusing nip part N, the fusing roller31or the pressure roller32takes a pressure applying posture (first posture). When the fusing roller31or the pressure roller32is in the pressure applying posture, the fusing roller31is in press contact with the pressure roller32by a first pressure, thereby forming the fusing nip part N with a predetermined nip width. By contrast, when the image forming apparatus1is under preparation for image formation (sleep mode), the fusing roller31or the pressure roller32takes a pressure reducing posture (second posture). When the operating panel19receives an instruction for image formation in the sleep mode of the image forming apparatus1, the operating mode of the image forming apparatus1is switched to the image forming mode to immediately set the image forming apparatus1to be in a state capable of performing image formation. In other words, when the operating mode of the image forming apparatus1is set in the sleep mode, the image forming apparatus1is in a standby state, and no sheet passes through the fusing nip part N. When the fusing roller31or the pressure roller32takes the pressure reducing posture, the fusing roller31is in press contact with the pressure roller32by a second pressure which is reduced from the first pressure. In the present embodiment, it is the posture of the fusing roller31that is changed between the pressure applying posture and the pressure reducing posture.

The nip pressure adjusting motor72generates drive power necessary for posture change between the pressure applying posture and the pressure reducing posture. It is noted that even when the fusing roller31is in the pressure reducing posture, the fusing nip part N is still formed as will be described later, so that the fusing roller31can follow rotation of the pressure roller32to be rotated.

The controller73is a microcomputer to control the operation of the fusing device30. The controller73reads out a program to operate so as to have functions of a roller drive controller74, a nip pressure adjusting section75(part of the adjustment mechanism), and a mode switching section76. The roller drive controller74controls the operation of the drive motor71to control each rotation of the fusing roller31and the pressure roller32. The nip pressure adjusting section75controls the operation of the nip pressure adjusting motor72to control posture change between the pressure applying posture and the pressure reducing posture. The mode switching section76performs control to switch the operating mode of the image forming apparatus1at least between the image forming mode (first mode) and the sleep mode (second mode).

When the mode switching section76switches the operating mode of the image forming apparatus1from the sleep mode to the image forming mode, the nip pressure adjusting section75drives the nip adjusting motor72to cause the fusing roller31to take the pressure applying posture. When the mode switching section76switches the operating mode of the image forming apparatus1from the image forming mode to the sleep mode, the nip pressure adjusting section75drives the nip adjusting motor72to cause the fusing roller31to take the pressure reducing posture.

When the mode switching section76sets the image forming mode, the roller drive controller74drives the drive motor71to rotate the fusing roller31and the pressure roller32at a first linear velocity. By contrast, when the mode switching section76sets the sleep mode, the roller drive controller74drives the drive motor71to rotate the fusing roller31and the pressure roller32at a second linear velocity lower than the first linear velocity.

Control by the roller drive controller74is performed to maintain a uniformly heated state of the fusing belt31A. If the fusing roller31stops in the sleep mode, only a part of the fusing belt31A, which faces the bobbin331, may be heated by the heater33of induction heating type. Where the image forming mode is set in this state, it may take time to stably distribute the heat in the circumferential direction of the fusing belt31A. By contrast, rotation of the fusing roller31even in the sleep mode can achieve uniform heating of the fusing belt31A. Accordingly, when the mode switching section76switches the operating mode of the image forming apparatus1from the sleep mode to the image forming mode, a sheet can be allowed to immediately pass through the fusing nip part N.

The hardware configuration of the fusing device30will be described next.FIG. 3is a perspective view of the fusing device30.FIG. 4is a perspective view of the fusing device30when viewed in a viewing direction different from that inFIG. 3.FIG. 5is an enlarged perspective view of a part V inFIG. 3. The fusing device30further includes a first conveyance roller34arranged in the vicinity of the outlet302and a second conveyance roller that forms a conveyance nip part together with the first conveyance roller34. The first conveyance roller34is rotatably held at the fuser housing300. The fuser housing300has a square pole shape long in the back-and-forth directions. An openable cover member351is mounted on the top of the fuser housing300.FIG. 3shows the state in which the cover member351opens the fuser housing300. The second conveyance roller35is held by the cover member351. As shown inFIG. 4, when the cover member351is closed, the second conveyance roller35is in contact with the first conveyance roller34.

The separation member40is a thin plate-shaped member substantially in a rectangular shape extending in the axial direction of the fusing roller31. In the vicinity of each opposite end of the fusing roller31, the separation member40is supported rotatably about its axis parallel to the rotational axis31S of the fusing roller31by the fuser housing300. As shown inFIG. 5, a tip end41A (tip end of plate-shaped member) of the separation member40(body portion41) faces the peripheral surface of the fusing roller31(fusing belt31A) so that a gap G is formed between the tip end41A and the peripheral surface of the fusing roller31.

When a sheet (not shown) having passed through the fusing nip part N reaches the gap G with it winding to the fusing roller31, the lead edge of the sheet abuts on the tip end41A of the separation member40. This separates the sheet from the fusing roller31. It is noted that a margin where no toner image is transferred is present in the lead edge part of a sheet in general. Accordingly, no toner layer acting as an adhesive layer is present on the lead edge part of a sheet. Therefore, the lead edge part of the sheet will not adhere to the fusing roller31. Thus, even if a sheet winds around the fusing roller31, the lead edge of the sheet floats up from the peripheral surface of the fusing roller31, so that the sheet can be separated from the fusing roller31even if the tip end41A of the separation member40(body portion41) does not abut on the peripheral surface of the fusing roller31.

Paired abutting members50are mounted on the respective opposite end parts of the separation member40(opposite end positions of the body portion41, which will be described later). The paired abutting members50form the gap G between the tip end41A of the separation member40(body portion41) and the peripheral surface of the fusing roller31. The abutting members51each include an abutting portion51. The abutting portion51protrudes more downward than the tip end41A of the separation member40and toward the center of the separation member40. The abutting portions51abut on the peripheral surface of the fusing roller31in the vicinity of the respective opposite ends of the fusing roller31to form the gap G. InFIG. 5, reference character T denotes an abutting point where the shown abutting portion51abuts on the peripheral surface of the fusing roller31.

FIG. 6is a schematic illustration showing the positional relationship between the abutting members50and the fusing roller31. A sheet passing region31C where a sheet (not shown) passes is set in the peripheral surface of the fusing roller31. Sheet non-passing regions where a sheet does not pass are set outside the opposite end parts of the sheet passing region31C. In other words, a sheet passing through the fusing nip part N will not come in contact with the peripheral surface of the fusing roller31in the sheet non-passing regions31E. The abutting members50abut on the sheet non-passing region31E in the peripheral surface of the fusing roller31to form the gap G between the tip end41A of the separation member40and the peripheral surface of the fusing roller31.

FIG. 7is a perspective view of the obverse side of the separation member40.FIG. 8is a perspective view of the reverse side (side facing the peripheral surface of the fusing roller31) of the separation member40. The separation member40includes a thin plate-shaped body portion41(plate-shaped member) extending in the axial direction of the fusing roller31. The separation member40further includes a holding plate42to hold the body portion41. The lower edge of the body portion41serves as the tip end41A. The holding plate42has a flat holding surface extending in the axial direction of the fusing roller31. The body portion41is held in a state adhering to the flat holding surface, and the tip end41A extends downward of the lower end of the holding plate42.

An end plate43is provided at each opposite end of the holding plate42. The end plate43will be discussed with reference toFIG. 16, which is an enlarged view of one end plate43, in addition toFIGS. 7 and 8.

The end plates43each are an end portion of the holding plate42which is bent orthogonally to the flat holding surface. The end plates43are supported by side plate frame members361of the fixed framework36. Each side plate frame member361forms a part of the fuser housing300. A support hole431(part of turning mechanism) is formed in each end plate43. Each support hole431is formed on the based end side (opposite side to the tip end41A) of the separation member40.

Support pins363(support portions or parts of turning mechanism) protruding from the side plate frame members361are inserted in the support holes431. The separation member40is supported by means of the support pins363so as to be rotatable about an axis in parallel to the axial direction of the fusing roller31. A torsion coil spring45(part of turning mechanism) is arranged in the vicinity of each support pin363. One end451of each torsion coil spring45engages with a window365perforated in the corresponding side plate frame member361. The other end452of the torsion coil spring45engages with a hook432of the corresponding end plate43. It is noted thatFIG. 16shows the state before the other end452engages with the hook432.

Each torsion coil spring45urges to rotate the separation member40about the axes of the support pins363in the direction where the tip end41A approaches the peripheral surface of the fusing roller31(direction indicated by the arrow R2inFIG. 16). This sets the tip end41A to be close to the peripheral surface of the fusing roller31when the fusing roller31is in the pressure applying posture. It is noted that the support hole431is an oblong hole. Even if the parallel positional relationship between the tip end41A of the separation member40and the peripheral surface of the fusing roller31deviates, the oblong support holes431can allow the position of the separation member40supported by the support pins363to be shifted to correct the deviation.

Protruding pieces44(parts of separation member which abut on stopper members) protrude from the upper parts of the respective end plates43. The protruding pieces44face stoppers362(isolation mechanism) provided on the respective side plate frame members361in a state in which the support pins363are inserted in the support holes431. The stoppers362are bent portions of the side plate frame members361. When the separation member40turns in the direction indicated by the arrow R2, the protruding pieces abut on the stoppers362. This can inhibit the separation member40from turning in the direction indicated by the arrow R2.

The abutting members50are mounted on the end parts411of the body portion41of the separation member40. The body portion41is wider than the holding plate42, while each end part411of the body portion41is substantially the same in width as the holding plate42. The butting members50are mounted on the holding plate42to extend toward the tip end41A from the end parts411.

The abutting members50will be described below with reference toFIG. 11as an enlarged cross sectional view of the separation member40in addition toFIGS. 7 and 8.

Each abutting member50includes the abutting portion51. The abutting member50further includes a trunk portion52in which a screw hole is formed and a positioning pin53protruding from one side of the trunk portion52. The abutting portion51is a tip end portion of a suspending piece54extending downward from the trunk portion52. A receiving hole (not shown) in which the positioning pin53is fitted is formed in the holding plate42. A screw hole (not shown) corresponding to the screw hole of the trunk52is also formed in the holding plate42. In addition, a mounting hole412is formed in a part of each end part411of the body portion41which corresponds to the screw hole of the trunk portion52and the screw hole of the holding plate42. When the positioning pins53are inserted in the receiving holes of the holding plate42, the holding plate42provisionally catches the abutting members50. At the provisional catch, the screw holes of the trunk portions52are positioned at the screw holes of the holding plate42. Then, the mounting screws421pass through the screw holes of the trunk portions52, the screw holes of the holding plate42, and the mounting holes412, thereby being inserted. Next, nuts (not shown) are screw inserted in the mounting screws421, thereby fixing the main body41and the abutting members50to the holding plate42.

With reference toFIGS. 9-17, description will be made next about posture change of the fusing roller31and turning of the separation member40and the abutting member50accompanied by the posture change.FIGS. 9 and 10are cross sectional views of the fusing device30. In detail,FIG. 9shows a state in which the fusing roller31is in the pressure applying posture.FIG. 10shows a state in which the fusing roller31is in the pressure reducing posture.

As shown inFIG. 9, when the fusing roller31is in the pressure applying posture, the rotational axis31S of the fusing roller31is located at a normal position relative to the rotational axis32S of the pressure roller32. When the rotational axis31S of the fusing roller31is located at the normal position, a fusing nip part N1is formed. The fusing nip part N1has a nip width necessary for fusing in the image forming mode. By contrast, as shown inFIG. 10, when the fusing roller31is changed in posture from the pressure applying posture to the pressure reducing posturer, the rotational axis31S shifts to a release position apart from the normal position relative to the rotational axis32S. When the rotational axis31S of the fusing roller31is located at the release position, a fusing nip part N2is formed. The fusing nip part N2has a minimum nip width that enables transmission of the rotational drive force of the pressure roller32to the fusing roller31in the sleep mode.

When the fusing nip part N1is formed, the fusing roller31is in press contact with the pressure roller32by high pressure to be deformed large. A high speed image forming apparatus of which linear velocity is high requires a wide fusing nip width. In turn, the degree of deformation of the fusing roller31is further increased. Continuation of deformation of the fusing roller31may compress and deform the roller member31B to change its original shape, thereby reducing the lifetime of the fusing roller31. In view of this, in the present embodiment, the fusing nip part N2shallower than the fusing nip part N1is formed during the time other than the time in the image forming mode. This can extend the lifetime of the fusing roller31.

FIG. 14is a perspective view of the fusing device30from which the fusing roller fixing roller31is removed.FIG. 15is a perspective view of the removed fusing roller31.FIG. 16is a perspective view of the main part inFIG. 14.FIG. 17is a cross sectional view taken along the line XVII-XVII inFIG. 14. The fuser housing300includes a fixed framework36and a movable framework37movable relative to the fixed framework36. The fixed framework36holds the pressure roller32rotatably about the rotational axis32S. The movable framework37holds the fusing roller31rotatably about the rotational axis31S. Moving the movable framework37relative to the fixed framework36can achieve posture change of the fusing roller31.

As shown inFIG. 14, the fixed framework36includes a pair of front and rear side plate frame members361that hold the pressure roller32. The stoppers362and the support pins363protrude from the paired side plate frames361. The space on the left side of the pressure roller32between the paired side plate frames361is a space where the fusing roller31is to be mounted. The movable framework37includes a pair of front and rear holding frame members371and a horizontal frame member372connected to the paired holding frame members371, as shown inFIG. 15. A shaft pin373protruding outward in the back-and-forth directions is provided on each holding frame member371. On the other hand, a pin receiving portion364to which the shaft pin373is to be inserted is formed in each side plate frame member361. The movable framework37is mounted on the fixed framework36so as to be rotatable about the shaft pins373pivotally supported in the pin receiving portions364. When the movable framework37is mounted, the side plate frame members361of the fixed framework36face the corresponding holding frame members371of the movable framework37in an adjacent manner.

A adjustment mechanism60including the nip pressure adjusting motor72and a gear mechanism is mounted on the front side plate frame member361of the fixed framework36.FIGS. 14 and 4show the state when covers38cover the side plate frame members361. Accordingly, the gear mechanism is not exposed inFIGS. 14 and 4. The adjustment mechanism60moves the movable framework37, that is, turns the movable framework37about the shaft pins373to move the rotational axis31S of the fusing roller31in parallel to the rotational axis32S of the pressure roller32. Thus, the posture of the fusing roller31is changed between the pressure applying posture and the pressure reducing posture. It is noted that the gear mechanism has a function of transmitting drive power that the nip pressure adjusting motor72generates to the movable framework37to turn the movable framework37about the shaft pins373.

When the fusing roller31is in the pressure applying posture, as shown inFIG. 9, the adjustment mechanism60sets the position of the movable framework37so that the rotational axis31S is located at the normal position relative to the rotational axis32S. When the posture of the fusing roller31is changed from the pressure applying posture to the pressure reducing posture shown inFIG. 10, the adjustment mechanism60turns (moves) the movable framework37about the shaft pins373in the direction indicated by the arrow R1to shift the rotational axis31S from the normal position to the release position relative to the rotational axis32S.

FIG. 11is an enlarged cross sectional view of the separation member40and one abutting member50. InFIG. 11, the solid line indicates the position of a peripheral surface31H of the fusing roller31(fusing belt31A) when the fusing roller31is in the pressure applying posture. By contrast, the two-dot chain line indicates the position of the peripheral surface31H of the fusing roller31when the fusing roller31is in the pressure reducing posture. When the fusing roller31is in the pressure applying posture, the columnar abutting portion51of each abutting member50abuts on the peripheral surface31H of the fusing roller31. This forms the gap G between the tip end41A of the separation member40and the peripheral surface31H of the fusing roller31.

By contrast, when the fusing roller31is in the pressure reducing posture, the abutting portion51of each abutting member51does not abut on but separates from the peripheral surface31H of the fusing roller31by a distance D1. This is because the separation member40does not fully follow the movement of the fusing roller31accompanied by posture change from the pressure applying posture to the pressure reducing posture. Specifically, an isolation mechanism is provided in the fusing device30. The isolation mechanism allows the abutting portion51of each abutting member50to abut on the peripheral surface31H of the fusing roller31when the fusing roller31is in the pressure applying posture (first posture), while separating each of the abutting portion51from the peripheral surface31H of the fusing roller31in the process of posture change of the fusing roller31from the pressure applying posture to the pressure reducing posture (second posture). When the abutting portion51of each abutting member50abuts on the peripheral surface31H of the fusing roller31, the tip end41A of the separation member40is close to the peripheral surface31H of the fusing roller31.

The stoppers362provide at the side plate frame members361function as the isolation mechanism in the present embodiment. As has been described above, the protruding pieces44are formed on the end plates43of the separation member40. The protruding pieces44are each located at a position where they can interfere with the corresponding stoppers362when the separation member40turns about the support pins361. The stoppers362interfere with the protruding pieces44in the process when the adjustment mechanism60moves the rotational axis31S of the fusing roller31from the normal position to the release position. This can separate the abutting portions51from the peripheral surface31H of the fusing roller31. This point of view will be described in detail.

FIGS. 12 and 13are perspective views showing the relationship between one stopper362and the corresponding protruding piece44.FIG. 12shows a state where the fusing roller31is in the pressure applying posture. In this state, a gap E is present between the stopper362and the protruding piece44, and they are out of contact with each other.FIG. 13shows a state where the fusing roller31is in the pressure reducing posture. In this state, the stopper362is in contact with the protruding piece44to restrict turning of the separation member40.

As shown inFIGS. 16 and 17, the torsion coil springs45urge to turn the separation member40about the axes of the support pins363in the direction indicated by the arrow R2. In other words, the torsion coil springs45urge the separation member40so that its tip end41A goes toward the peripheral surface31H of the fusing roller31. Thus, the abutting portions51of the abutting members50are pushed against the peripheral surface31H of the fusing roller31when the fusing roller31is in the pressure applying posture. By contrast, when the rotational axis31S of the fusing roller31moves from the normal position to the release position to move the peripheral surface31H of the fusing roller31in the direction indicted by the arrow R1(FIG. 9), the separation member40is turned about the axes of the support pins363in the direction indicated by the arrow R2by the urging force of the torsion coil springs45so that the tip end41A follows the movement of the rotational axis31S of the fusing roller31.

However, when the separation member40turns in the direction indicted by the arrow R2to some extent, the protruding pieces44interfere with the stoppers362. That is, only by the gap E between the stoppers362and the protruding pieces44, the separation member40can turn in the direction indicated by the arrow R2to allow the tip end41A to follow the movement of the peripheral surface31H of the fusing roller31, as shown inFIGS. 16 and 17. However, upon interference of the protruding pieces44with the stoppers362, the separation member40following the movement of the peripheral surface31H of the fusing roller31is inhibited from turning. The adjustment mechanism60moves the movable framework37even after the protruding pieces44interfere with the stoppers362. Accordingly, the abutting portions51of the abutting members50are separated from the peripheral surface31H of the fusing roller31.

As described above, in the present embodiment, the stoppers362inhibit the movement of the tip end41A of the separation member40following the movement of the peripheral surface31H of the fusing roller31(turning of the separation member40) in the process when the rotational axis S31of the fusing roller31shifts from the normal position to the release position. This can separate the abutting members50from the fusing roller31. In other words, shift of the rotational axis31S of the fusing roller31is linked to separation of the abutting members50from the peripheral surface31H of the fusing roller31, thereby ensuring separation of the abutting members50from the peripheral surface31H of the fusing roller31. Further, the simple configuration in which the protruding pieces44of the separation member40abut on the stoppers362can link the shift of the rotational axis31S of the fusing roller31to the separation of the abutting members50from the fusing roller31.

The image forming apparatus1according to the present embodiment has the above described configuration to cause the abutting members50not to always abut on the peripheral surface31H of the fusing roller31(fusing belt31A). That is, when the fusing roller31is in the pressure applying posture in the image forming mode, the abutting portions51of the abutting members50abut on the peripheral surface31H of the fusing roller31to form the gap G between the tip end41A and the peripheral surface31H of the fusing roller31. By contrast, in the process of posture change of the fusing roller31to the pressure reducing posture in the sleep mode, the abutting portions51separate from the peripheral surface31H of the fusing roller31. As has been described above, the roller drive controller74rotates the fusing roller31even in the sleep mode to uniformly heat the fusing belt31A. While, the abutting portions51do not abut on the peripheral surface31H of the fusing roller31in the sleep mode. Accordingly, in the sleep mode, the peripheral surface31H of the fusing roller31can be prevented from being worn out. Wearing out of the fusing roller31may be caused by abutment of the abutting portions51on the peripheral surface31H of the fusing roller31. Accordingly, wearing out of the fusing belt31A can be reduced as a whole at the abutting points thereof on the fusing belt31A. Thus, the lifetime of the fusing belt31A can be extended.

Preferable arrangement of the stoppers362, in other words, preferable setting of the gap E will be described next.FIGS. 18A and 18Bare schematic illustrations for explaining press contact between the fusing roller31and the pressure roller32. The fusing roller31includes the elastic roller member31B and the fusing belt31A, as shown inFIG. 18A. The fusing belt31A is fitted around the roller member31B with clearance C left. The clearance C is effectively present in a non-heating state although it substantially disappears when the roller member31B is heated to be thermally expanded.

By contrast, as shown inFIG. 18B, the pressure roller32presses and deforms the roller member31B. When the fusing roller31is in press contact with the pressure roller32, the peripheral surface of the fusing roller31is recessed in an arc shape to form the fusing nip part N with a predetermined nip width. When the fusing roller31is in press contact with the pressure roller32, the presence of the clearance C results in formation of bulges each of which is a part of the fusing belt31A swelling on the peripheral surface of the roller member31B.

FIGS. 19A and 18Bare schematic illustrations for explaining the relationship between the bulges and one abutting member50.FIGS. 19A and 19Bshow the process of the fusing roller31gradually pushing the pressure roller32.

FIG. 19Ashows a state in which a fusing nip part Na with a contact depth T1is formed. The state of the fusing roller31being in the pressure reducing posture transfers to the state of it pushing the pressure roller32, thereby forming the fusing nip part Na. The contact depth T1of the fusing nip part Na is larger than the contact depth of the fusing nip part N2formed when the fusing roller31is in the pressure reducing posture and is smaller than the contact depth of the fusing nip part N1formed when the fusing roller31is in the pressure applying posture.

At this time, bulges B1, each of which is a part of the fusing belt31A swelling in a convex shape, are formed on each opposite side of the fusing nip part Na in the circumferential direction of the fusing roller31. Each bulge B1is formed in a manner that the clearance C is compressed at the fusing nip part Na to displace the space of the clearance C sideward of the fusing nip part Na. A hollow F in a convex shape is formed in the back of each bulge B1. Accordingly, the bulge B1cannot be supported by the peripheral surface of the roller member31B. The bulge B1swells gradually largely as pushing of the fusing roller31progresses.

Thereafter, as pushing of the fusing roller31against the pressure roller32further progresses to increase the size of each bulge B1to some extent, the bulge B1moves away from the fusing nip part Na in the circumferential direction of the fusing roller31.FIG. 19Bshows a state when pushing of the fusing roller31progresses until a fusing nip part Nb with a contact depth T2deeper than the contact depth T1is formed. The contact depth T2of the fusing nip part Nb approximates the contact depth of the fusing nip part N1formed when the fusing roller31is in the pressure applying posture. The comparatively large hollow F is formed on the back of each grown bulge B2. The hollow F (bulge B2) moves away from the fusing nip part Nb.

The abutting portion51of each abutting member50abuts on the peripheral surface of the fusing roller31at the predetermined abutting point51T. When the abutting portion51abuts on the corresponding bulge B2, which is not supported by the peripheral surface of the roller member31B, the fusing belt31A may be damaged. Specifically, when a force of the abutting portion51coming in face contact with the abutting point51T acts on the bulge B2in the process when the bulge B2passes on the abutting point51T, the abutting portion51may squeeze the fusing belt31A into the hollow F. As a result, the fusing belt31A may be damaged.

In view of the above drawback, the stoppers362are preferably provided so that the abutting portions51of the abutting members50abut on the peripheral surface of the fusing belt31A at the abutting points51T after one of the bulges B2, which is located downstream in the direction of rotation of the fusing roller31, passes on the abutting points51T in the process when the rotational axis31S of the fusing roller31moves from the release position to the normal position. In other words, it is desirable that the gap E between the stoppers362and the protruding pieces44is set so that the abutting portions51come in face contact with the peripheral surface of the fusing roller31at the abutting points51T after the bulges B1are formed on the sides of the fusing nip part Na, as shown inFIG. 19A, and then, the one bulge B2passes on the abutting points51T as shown inFIG. 19B. This can avoid abutting of the bulge B2swelling out as the hollow F on the abutting members50, thereby obviating damage of the fusing belt31A, which may be caused by interference of the abutting members50with the bulge B2.

As described so far, according to the fusing device30and the image forming apparatus1of the present embodiment, the abutting members50are separated from the peripheral surface of the fusing roller31in the process when the fusing roller31changes in posture from the pressure applying posture to the pressure reducing posture, so that the abutting members50do not always abut on the peripheral surface of the fusing roller31. Accordingly, wearing out of the peripheral surface of the fusing roller31, which may be accompanied by abutment of the abutting members50on the peripheral surface of the fusing roller31, can be reduced, thereby extending the lifetime of the fusing belt31A. This can achieve extension of the lifetime of the fusing roller31that forms the fusing nip part N.

Further, according to the fusing device30and the image forming apparatus1of the present embodiment, the abutting members50are separated from the peripheral surface of the fusing roller31in the process when the rotational axis31S of the fusing roller31moves from the normal position to the release position. In other words, the movement of the movable framework37(shift of the rotational axis31S of the fusing roller31) is linked to the movement of the abutting members50, which can reliably separate the abutting members50from the peripheral surface of the fusing roller31.

Moreover, according to the fusing device30and the image forming apparatus1of the present embodiment, parts (protruding pieces44) of the separation member40come in contact with the stoppers362when the rotational axis31S of the fusing roller31moves from the normal position to the release position, thereby inhibiting the separation member40from turning. This can separate the abutting members50from the peripheral surface of the fusing roller31. Thus, the isolation mechanism (mechanism for separating the abutting members50from the peripheral surface of the fusing roller31) can be achieved with a simple configuration.

Still further, according to the fusing device30and the image forming apparatus1of the present embodiment, parts (protruding pieces44) of the separation member40come in contact with the stoppers362when the rotational axis31S of the fusing roller31moves from the normal position to the release position, thereby inhibiting the separation member40urged by the torsion coil springs45from turning. This can separate the abutting members50from the peripheral surface of the fusing roller31. Thus, the isolation mechanism (mechanism for separating the abutting members50from the peripheral surface of the fusing roller31) can be achieved with a simple configuration.

Yet further, according to the fusing device30and the image forming apparatus1of the present embodiment, the fusing roller31includes the elastic roller member31B and the fusing belt31A fitted around the roller member31B. The pressure roller32presses and deforms the roller member31B of the fusing roller31. The abutting members50abut on the peripheral surface of the fusing belt31A. With this configuration, wearing out of the peripheral surface of the fusing belt31A, which may be accompanied by abutting of the abutting members50on the peripheral surface of the fusing belt31A, can be reduced. Thus, the lifetime of the fusing belt31A can be extended.

Furthermore, according to the fusing device30and the image forming apparatus1of the present embodiment, the abutting members50can be prevented from abutting on the bulge which cannot be supported by the peripheral surface of the roller member31B. Accordingly, damage to the fusing belt31A, which may be caused due to interference of the abutting members50with the bulge, can be obviated.

Still further, according to the fusing device30and the image forming apparatus1of the present embodiment, the fusing roller31rotates not only in the pressure applying posture but also in the pressure reducing posture. Accordingly, the heater33of induction heating type can be prevented from heating only the same part of the fusing roller31, thereby achieving uniform heating of the fusing roller31. Specifically, if the abutting members50abut on the peripheral surface of the fusing roller31even when the fusing roller31is in the pressure reducing posture, wearing out of the fusing roller31may progress. However, in the present embodiment, since the abutting members50are out of contact with the peripheral surface of the fusing roller31when the fusing roller31is in the pressure reducing posture, the fusing roller31can be prevented from being worn out.

In addition, according to the fusing device30and the image forming apparatus1of the present embodiment, the fusing roller31rotates even when the image forming apparatus1is in the sleep mode as a standby state for image formation. Accordingly, the heater33of induction heating type can be prevented from heating only the same part of the fusing roller31, thereby achieving uniform heating of the fusing roller31. Accordingly, when the mode switching section76switches the operating mode of the image forming apparatus1from the sleep mode to the image forming mode, a sheet can be allowed to immediately pass through the fusing nip part N. Specifically, if the abutting members50abut on the peripheral surface of the fusing roller31even when the fusing roller31is in the pressure reducing posture, wearing out of the fusing roller31may progress. However, in the present embodiment, since the abutting members50are out of contact with the peripheral surface of the fusing roller31when the fusing roller31is in the pressure reducing posture, the fusing roller31can be prevented from being worn out.

The embodiment of the present disclosure has been described so far. However, the present disclosure is not limited to the above embodiment and can be altered as follows, for example.

As the first roller, the fusing roller31of so-called single axis type, which is formed in a fashion that the fusing belt31A covers the single roller member31B, is exemplified in the above embodiment. The first roller may be a fusing roller of a so-called dual axis type, which is formed in a fashion that a fusing belt is wound between an elastic roller member and a heating roller arranged in close proximity to the heater33.

The above embodiment describes an example in which the fusing roller31is mounted on the movable framework37, while the pressure roller32is mounted on the fixed framework36. Rather, the pressure roller32may be mounted on the movable framework37, and the fusing roller31may be mounted on the fixed framework36.

In the above embodiment, the protruding pieces44of the separation member40urged by the torsion coil springs45abut on the stoppers362as the isolation mechanism and stop. However, this is merely one example, and the isolation mechanism may be a mechanism in which a claw provided at the separation member40fits into a groove formed in the fixed framework36. Alternatively, a retractable member that moves in association with movement of the movable flame37may inhibit the separation member40from turning.