Fixing device provided with pressure belt

A fixing device includes a fixing roller including a heat generating part, an endless belt facing the fixing roller, a first supporter which supports the belt to cause the belt to face the fixing roller at a first position, a second supporter which supports the belt at a downstream side in the rotation direction of the belt to cause the belt to face the fixing roller at a second position, and a pressure applying member having a center position, configured to urge the belt toward the fixing roller to form a nip between the fixing roller and the belt, wherein a first distance between the center position and the first position is longer than a second distance between the center position and the second position.

FIELD

Embodiments described herein relate generally to a fixing device used in an image forming apparatus.

BACKGROUND

In an image forming apparatus such as a copying machine or a printer, there is a fixing device in which, for the purpose of improving an environmental property, a belt member is used to widen a nip width between a heating member and a pressure member in order to start the apparatus at a high speed and to perform fixing at a low temperature. In the fixing device in which the belt member is used to widen the nip width, there is a fear that toner is shifted on a sheet during fixing by the deflection, floating or the like of the belt member, and an image is blurred.

DETAILED DESCRIPTION

In general, according to one embodiment, a fixing device includes a fixing roller including a heat generating part, an endless belt facing the fixing roller, a first supporter which supports the belt to cause the belt to face the fixing roller at a first position, a second supporter which supports the belt at a downstream side in the rotation direction of the belt to cause the belt to face the fixing roller at a second position, and a pressure applying member having a center position, configured to urge the belt toward the fixing roller to form a nip between the fixing roller and the belt, wherein a first distance between the center position and the first position is longer than a second distance between the center position and the second position.

Hereinafter, embodiments will be described.FIG. 1is a schematic structural view showing an MFP (Multi Function Peripheral)10as an example of an image forming apparatus including a fixing device31of an embodiment. The MFP10includes a printer part11as an image forming part, a scanner part12, a paper feed part13and a paper discharge part22. The MFP10includes a CPU100to control the printer part11, the scanner part12, the paper feed part13and the paper discharge part22.

The paper feed part13includes a first and a second paper feed cassette13aand13b. The paper feed cassettes13aand13bcan feed both an unused sheet and a reuse sheet (a sheet obtained by erasing an image by a color erasing process).

The printer part11includes a charging device16to uniformly charge a photoconductive drum14rotating in an arrow m direction, and a laser exposing device17to form an electrostatic latent image on the photoconductive drum14by irradiating the charged photoconductive drum14with a laser beam17abased on image data from the scanner part12. The printer part11includes a developing device18to supply toner to the electrostatic latent image on the photoconductive drum14, a transfer device20to transfer a toner image formed on the photoconductive drum14to a sheet P as a recording medium, and a cleaner21.

The developing device18uses a two-component developer as a mixture of toner and magnetic carrier and supplies the toner to the electrostatic latent image on the photoconductive drum14. The toner is, for example, a color erasable colorant and is a color erasable toner whose color can be erased by heating at a specific temperature. The color erasable toner contains a binder resin, a coloring compound as a colorant and a developer. If the toner image formed using the color erasable toner is heated at the specific temperature, the coloring compound and the developer in the toner are dissociated, and the color of the toner image is erased. For example, the color erasable toner is fixed to a sheet at a relatively low temperature of about 80 to 100° C., and the color is erased at a relatively high temperature of 180 to 200° C. The developing device18may use a color un-erasable toner whose color is not erased even if the toner is heated to the specific temperature.

The photoconductive drum14, the charging device16, the laser exposure device17, the developing device18, the transfer device20and the cleaner21constitute the image forming part. The printer part11includes the fixing device31between the photoconductive drum14and the paper discharge part22.

The scanner part12includes an optical mechanism26ato optically read a document on a document table23, and a charge coupled device26bto convert a light signal from the optical mechanism26ainto an electric signal.

The MFP10includes, between the paper feed part13and the photoconductive drum14, a conveyance mechanism28, a register roller pair30to convey the sheet P to between the photoconductive drum14and the transfer roller20in synchronization with the toner image on the photoconductive drum14. The MFP10includes a paper discharging roller32to discharge the sheet P to the paper discharge part22after fixing.

By the structure as stated above, the MFP10transfers the color erasable toner image formed by the printer part11to the sheet P fed from the paper feed part13. The MFP10fixes the sheet P having the color erasable toner image by the fixing device31, and discharges the sheet to the paper discharge part22after completion of printing. The image forming apparatus is not limited to this. The image forming apparatus may include plural printer parts, that is, a printer part using a color erasable toner and a printer part using a color un-erasable toner. The print system of the printer part is not limited to the electrophotographic system, and may be an inkjet system or the like.

The fixing device31will be described in detail. As shown inFIG. 2, the fixing device31includes a heat roller37as a fixing roller that contacts the sheet P having a toner image, and a press belt38as a belt. The fixing device31nips and conveys the sheet P by a nip40formed between the heat roller37and the press belt38, and heats, presses and fixes the toner image to the sheet P.

The heat roller37is such that a mold release layer is coated on a hollow aluminum roller. The heat roller37includes a first halogen lamp41and a second halogen lamp42which are a heat generating part and have the same heat generation amount in the hollow inside of the heat roller37. For example, the luminous intensity distribution area of the first halogen lamp41is a center area in an axial direction of the heat roller37, and the luminous intensity distribution area of the second halogen lamp42is side areas on both sides of the center area. The first halogen lamp41and the second halogen lamp42heat the whole area of the heat roller37in the axial direction.

The press belt38is stretched by a belt heat roller46, an exit roller47and a tension roller48, and rotates in an arrow v direction. A housing43to support the press belt38presses the press belt38to the heat roller37by a belt press mechanism44. A nip pad50as a pressure applying member to press the press belt38to the heat roller37side exists between the belt heat roller46inside the press belt38and the exit roller47.

The belt heat roller46as a first support part and as a first roller is such that a mold release layer is coated on a surface of a hollow aluminum roller. The belt heat roller46includes a third halogen lamp51as a second heat generating part in the hollow inside of the belt heat roller46. In the third halogen lamp51, the whole area of the belt heat roller46in the axial direction is a luminous intensity distribution area, and the whole area of the belt heat roller46is heated. The belt heat roller46is close to the heat roller37. The press belt38contacts the heat roller37at a first position T1.

The exit roller47as a second support part and as a second roller includes a solid rubber around a core metal made of Steel Use Stainless (SUS). The exit roller47causes the press belt38to face and contact the heat roller37at a second position T2. As compared with the belt heat roller46, the diameter of the exit roller47is small, and assists peeling of the sheet P from the heat roller37after the sheet passes through the nip40. The tension roller48includes a PFA tube covered on the periphery of a core metal made of SUS. The fixing device31has a large nip width from the first position T1to the second position T2.

For example, the exit roller47is connected to a motor49as a belt driving part. At the time of fixing, the press belt38is rotated in the arrow v direction by driving of the motor49through the exit roller47. The exit roller47is driven to rotate the press belt38, so that the deflection of the press belt38in the nip40is suppressed. The press belt38may be rotated by driving the belt heat roller46by the motor.

The heat roller37which comes in press contact with the press belt38is driven by the press belt38and is rotated in an arrow w direction. In the fixing device31, the heat roller37may be driven instead of driving the press belt38. The press belt38which comes in press contact with the heat roller37may be dependently driven by providing a motor as a fixing driving part to drive the heat roller37.

The fixing device31includes a roller thermistor37ato detect the temperature of the heat roller37and a belt thermistor38ato detect the temperature of the press belt38. The CPU100controls the temperature of the heat roller37or the press belt38according to the temperature detection result obtained by the roller thermistor37aor the belt thermistor38a.

The nip pad50is such that a silicone rubber50ais bonded to an auxiliary plate50b. For example, a slide sheet having a high sliding property and high wear resistance may be made to intervene between the nip pad50and the press belt38. The slide sheet is made to intervene to reduce the friction between the nip pad50and the press belt38.

The nip pad50is symmetrical with respect to a center position52as a center in the rotation direction of the press belt38. In the rotation direction of the press belt38, the first position T1is located at the upstream side of the nip pad50, and the second position T2is located at the downstream side of the nip pad50. If a first distance between the center position52and the first position T1is α, and a second distance between the center position52and the second position T2is β, the nip pad50is located at a position where α>β is established. The nip pad50includes a nip press mechanism53to press the nip pad50toward the heat roller37independently of the belt press mechanism44.

The housing43supports the nip pad50and the nip press mechanism53to enable reciprocating movement in the rotation direction of the press belt38. The nip pad50and the nip press mechanism53are slid along the heat roller37in the inside of the press belt38, so that the first distance α and the second distance β are changed.

The fixing device31starts a warm-up operation by turning on power supply. During the warm-up operation, the CPU100controls so that the belt press mechanism44presses the housing43toward the heat roller37, the nip press mechanism53presses the nip pad50toward the heat roller37, the first to the third halogen lamps41,42and51are turned on, and the motor49is driven. The heat roller37is driven by the rotation of the press belt38in the arrow v direction and is rotated in the arrow w direction.

If the warm-up is completed and a ready mode occurs, the CPU100controls the first to the third halogen lamps41,42and51according to the detection result of the roller thermistor37aor the belt thermistor38a, and keeps the fixing device31at a ready temperature. During the ready mode, the CPU100controls the nip press mechanism53, and reduces the pressing force of the nip pad50to the heat roller37to the pressure in the ready mode. The pressing force of the nip pad50is reduced to prevent the heat roller37or the nip pad50from being deformed.

If the MFP10starts printing, the fixing device31nips and conveys the sheet P having a toner image from the first position T1to the second position T2where the heat roller37and the press belt38contact each other, and heats, presses and fixes the toner image to the sheet P. The CPU100controls the first to the third halogen lamps41,42and51, and keeps the fixing device31at the fixing temperature. During the printing operation, the CPU100controls the nip press mechanism53, and raises the pressure of the nip pad50to the heat roller37up to the fixing pressure. Since the image on the sheet P is formed of the color erasable toner, the fixing temperature of the fixing device31is relatively low. Since the nip width formed between the heat roller37and the press belt38extends from the first position T1to the second position T2and can be set to be large, even if the fixing temperature of the fixing device31is set to be low, high fixing performance can be obtained.

If the printing is ended, the CPU100controls the nip press mechanism53to reduce the pressure of the nip pad50to the heat roller37, and keeps the fixing device31at the ready temperature.

Evaluation of the fixing performance of the fixing device31will be described.

In the fixing device31, the first distance α and the second distance β between the first position T1and the second position T2of the heat roller37and the press belt38were changed and shift of an image was evaluated.

As shown inFIG. 3, the evaluation was made such that a lattice image60was printed on a sheet P of A4 size of JIS standard, and shift amounts were measured at four points in an arrow z direction, which is the conveyance direction of the sheet P, and at five points in the direction perpendicular to the conveyance direction of the sheet P, that is, at 4×5=20 points (1) to (20) in total. The four points in the conveyance direction of the sheet P are the first line, the second line, the tenth line and the twentieth line of the lattice image60from an upper edge A of the sheet P. The five points in the direction perpendicular to the conveyance direction of the sheet P are 55 mm, 125 mm, 175 mm, 225 mm and 255 mm from a side edge B of the sheet P.

(1) The first distance α and the second distance β between the first position T1and the second position T2of the heat roller37and the press belt38were set to be α>β and the shift evaluation was performed.FIG. 4shows the result. In the case of α>β, image shift amounts at all the measurement points could be suppressed to 0.05 mm or less, the image shift could not be visually seen, and high fixing performance was obtained.

(2) Next, the nip pad50and the nip press mechanism53were moved in an arrow x direction ofFIG. 2with respect to the housing43, and the first distance α and the second distance β between the first position T1and the second position T2of the heat roller37and the press belt38were set to be α<β and the shift was evaluated.FIG. 5shows the result. In the case of α<β, the image shift is large especially at the leading edge of the sheet P. Besides, even at the tenth line or the twentieth line from the leading edge, the image shift is large as compared with the case of α>β.

From the evaluation of the image shift of the above (1) and (2), it was found that in the fixing device31, if the first distance α and the second distance β between the first position T1and the second position T2of the heat roller37and the press belt38were set to be α>β, the occurrence of the image shift was suppressed.

Incidentally, instead of driving the press belt38by the motor49to cause the heat roller37to be driven in the fixing device31, as shown in a modified example ofFIG. 6, in a fixing device71in which a heat roller37is driven by a fixing motor70to cause a press belt38in press contact with the heat roller37to be driven, image shift evaluation similar to the above (1) and (2) was performed.

(3)FIG. 7shows the result of the shift evaluation in which the press belt38is dependently driven, and the first distance α and the second distance β of the heat roller37and the press belt38are set to be α>β. In the case of α>β, the image shift amount could be suppressed to 0.05 mm or less at all measurement points, the image shift could not be visually seen, and high fixing performance was obtained.

(4) Next,FIG. 8shows the result of the shift evaluation in which the press belt38is dependently driven, and the first distance α and the second distance β of the heat roller37and the press belt38are set to be α<β. In the case of α<β, the image shift is large especially at the leading edge of the sheet P. Besides, even at the tenth line or the twentieth line from the leading edge, the image shift is large as compared with the case of α>β.

From the image shift evaluation of the above (3) and (4), it was found that in the fixing device71, if the first distance α and the second distance β between the first position T1and the second position T2of the heat roller37and the press belt38were set to be α>β, the occurrence of the image shift was suppressed.

According to the embodiment, the nip pad50is provided between the first position T1and the second position T2of the press belt38of the fixing device31. In the rotation direction of the press belt38, the first distance α at the upstream side of the center position52of the nip pad50and the second distance β at the downstream side of the center position52are set to be α>β. Even if the nip width of the fixing device31is set to extend from the first position T1to the second position T2and is set to be large. The image shift occurring at the time of fixing is suppressed, and high fixing performance is obtained. Since the image shift is suppressed, and the nip width of the fixing device31is set to be large, even if an image is formed by color erasable toner having a low fixing temperature, there is no image shift and high fixing performance is obtained.

According to the embodiment and the modified example, in both the fixing device31in which the press belt38is driven to rotate the heat roller37and the fixing device71in which the heat roller37is driven to rotate the press belt38, if α>β is set, the image shift occurring at the time of fixing is suppressed, and high fixing performance is obtained.