Fixing device and image forming apparatus

A fixing device comprises a heating roll; an endless belt; and a pressurizing member that forms a pressure contact portion for pushing the endless belt against an outer peripheral surface part of the heating roll from an inner peripheral surface side of the pressurizing member so that a recording medium holding an unfixed image passes between the pressure contact portion and the heating roll, wherein the pressurizing member includes: a hard pressurizing member and a soft pressurizing member the hard pressurizing member, and the soft pressurizing member are disposed in a state in which a clearance is present between the hard pressurizing member and the soft pressurizing member in a specific region part so that at least a specific recording medium passes in a fixing region of the pressure contact portion in the rotating axis direction of the heating roll.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2009-078844 filed on May 27, 2009.

BACKGROUND

1. Technical Field

The present invention relates to a fixing device and an image forming apparatus.

2. Related Art

In an image forming apparatus such as a printer, a copying machine or a facsimile, an unfixed image formed by a development through a developer is transferred onto a recording medium such as a paper, and the unfixed image thus transferred is heated and pressurized and is fixed onto the recording medium by a fixing device so that an image such as a character, a graphic, a pattern or a photograph image is formed.

The fixing device to be used in the image forming apparatus includes a belt fixing device having the following structure. The belt fixing device includes a heating rotor having a roll configuration to be heated by heating unit and to be thus rotated, an endless belt to be rotated in contact with an outer peripheral surface part in a direction of a rotating axis of the heating rotor, and a pressurizing member forming a pressure contact portion for pushing the endless belt against the outer peripheral surface part of the heating rotor to cause a recording medium having an unfixed image held thereon to pass between the pressure contact portion and the heating rotor.

In the belt fixing device of this type, the recording medium having the unfixed image held thereon is introduced into the pressure contact portion formed between the heating rotor and the endless belt and is caused to pass, and the unfixed image is thus heated and pressurized and is fixed onto the recording medium.

SUMMARY

According to an aspect of the invention, a fixing device comprises: a heating roll that includes an outer peripheral surface having an elastic layer formed thereon and is heated by a heating unit and is rotated; an endless belt to be rotated in contact with an outer peripheral surface part in a rotating axis direction of the heating roll; and a pressurizing member that forms a pressure contact portion for pushing the endless belt against the outer peripheral surface part of the heating roll from an inner peripheral surface side of the pressurizing member so that a recording medium holding an unfixed image passes between the pressure contact portion and the heating roll, wherein the pressurizing member includes: a hard pressurizing member disposed in a position on a downstream side in a passing direction of the recording medium in the pressure contact portion and having a higher hardness than that of the elastic layer of the heating roll; and a soft pressurizing member disposed in a position on an upstream side in the passing direction from the hard pressurizing member and having a lower hardness than that of the elastic layer of the heating roll to carry out an elastic deformation, and the hard pressurizing member and the soft pressurizing member are disposed in a state in which a clearance is present between the hard pressurizing member and the soft pressurizing member in a specific region part so that at least a specific recording medium passes in a fixing region of the pressure contact portion in the rotating axis direction of the heating roll.

DETAILED DESCRIPTION

A configuration for carrying out the invention (which will be hereinafter referred to as an “embodiment”) will be described below with reference to the drawings.

An image forming apparatus1is constituted as a color printer for forming a multicolor image and a monochrome (black-and-white) image, for example. As shown inFIG. 1, an imaging device2, a paper transport device3and a fixing device4are mainly provided in an internal space of a housing which is not shown. The imaging device2serves to form a toner image to be developed with a toner (colored fine powder) to be a dry developer based on input image data and to finally transfer the toner image onto a recording medium9such as a paper. The paper transport device3serves to transport the recording medium9to pass through a transfer position of each imaging device2. The fixing device4serves to cause the recording medium9having the toner image transferred thereto to pass, thereby fixing the toner image. A one-dotted chain line having an arrow in the drawing indicates a main transporting path of the recording medium9.

The imaging device2is constituted by four imaging devices20Y,20M,20C and20K for exclusively forming toner images having four colors of yellow (Y), magenta (M), cyan (C) and black (K) by utilizing a recording method such as a well-known electrophotographic method. Moreover, the four imaging devices20(Y, M, C, K) are arranged in series in a vertical direction, for example.

Each of the imaging devices (20Y,20M,20C,20K) basically includes a photosensitive drum21to be rotated and driven in a direction shown in an arrow (a counterclockwise direction in the drawing) and has a structure in which the following devices are mainly disposed around the photosensitive drum21. The main devices includes a charging device22for charging a surface (an image holding surface) of the photosensitive drum21into a predetermined potential, an exposing device23for irradiating a light H based on the four-color components subjected to a color separation of image data (a signal) onto the surface of the charged photosensitive drum21, thereby forming an electrostatic latent image (having each color component) with a potential difference, a developing device24(Y, M, C, K) for developing the electrostatic latent image having each color component with a toner having a corresponding color (Y, M, C, K), thereby forming a toner image, and a transferring device25having a transferring roll for transferring the toner image onto the recording medium9transported by means of (a paper transport belt31) of the paper transport device3.

For example, the photosensitive drum21is obtained by forming the image holding surface having a photosensitive layer (a photoconductive layer) constituted by an organic photosensitive material on a peripheral surface of a cylindrical substrate which is grounded. For the charging device22, there is used a contact charging method of applying a charging voltage to a charging roll to be rotated in contact with the surface of the photosensitive drum21, thereby carrying out charging. The exposing device23to be used is constituted by an LED (a light emitting diode) type recording head and a semiconductor laser scanning device. An image signal is input to the exposing device23. The image signal is obtained by carrying out a predetermined processing, through an image processing device (not shown), over image information input from an external apparatus serving as an image creating source, for example, an image reading device, a storing medium reading device or a computer which is provided in or connected to (including a radio communication) to the image forming apparatus1.

For the developing device24to be used, a developer (a single component developer or a two-component developer) containing a toner having a predetermined color is supplied in a charging state to the surface of the photosensitive drum21through a developing roll24ato which a developing voltage is applied. For the transferring device25, there is used a contact type for applying a transferring voltage to the transferring roll to be rotated in contact with the surface of the photosensitive drum21, thereby carrying out a transferring operation.

The paper transport device3is mainly constituted by the paper transport belt31, a plurality of supporting rolls32and33, an adsorbing roll35, and a belt cleaning device36. The paper transport belt31is rotated in a direction shown in an arrow (a counterclockwise direction in the drawing) while passing through a portion (a transferring position) between the photosensitive drum21of the imaging device2(20) and the transferring device25. The supporting rolls32and33rotatably support the paper transport belt31which is wrapped in a desired condition.

The adsorbing roll35serves to electrostatically adsorb the papers9supplied one by one from a paper feeding device (not shown) onto an outer peripheral surface of the paper transport belt31. The belt cleaning device36serves to remove a stuck substance such as an unnecessary toner or paper powder stuck to the outer peripheral surface of the paper transport belt31. For the paper transport belt31to be used, a material having a resistance regulating agent such as carbon dispersed in a predetermined amount into a synthetic resin such as a polyimide resin or a polyamide resin is utilized to be molded into a belt configuration taking an endless shape in a predetermined thickness. The supporting roll32is constituted as a driving roll and is rotated by a rotating power transmitted from a rotating and driving device including a motor which is not shown. A paper adsorbing voltage is applied from a power device (not shown) to the adsorbing roll35at a time that the paper9is adsorbed.

The fixing device4has a heating rotor5and a pressurizing rotor6provided in a housing41. The heating rotor5takes a roll configuration in which it is heated to hold a surface temperature into a predetermined temperature by heating unit and is rotated in a direction shown in an arrow. The pressurizing rotor6takes a belt configuration in which it is rotated while forming a pressure contact portion (a fixation processing portion) NP to come in contact with a surface part almost in a direction of a rotating axis of the heating rotor5at a predetermined pressure. The reference numeral39inFIG. 1denotes a discharging roll pair for discharging the recording medium9subjected to the fixation from the fixing device4. The details of the fixing device4will be described below.

The paper feeding device is mainly constituted by at least one housing cassette and a transmitting device. The housing cassette accommodates a plurality of recording media9having a predetermined size and type which is to be supplied to the imaging device2(20) in a stacking state. The transmitting device transmits and transports the recording media9accommodated in the housing cassette one by one. A transporting path for transporting the recording medium9is formed between the paper feeding device and the imaging device2. Although the recording medium9is not particularly restricted if a transport in an image forming apparatus and a transfer of a toner image can be carried out, there is mainly used a sheet-like recording medium such as a paper, a thick paper, a transparent sheet, a postal card or an envelope.

A basic image formation (print) is carried out by the image forming apparatus1in the following manner. Description will be given to a basic image forming operation to be carried out when forming a full color image which is constituted in combination of the toner images having the four colors (Y, M, C, K) over a single side of the recording medium9.

Upon receipt of an instruction for starting a printing operation in the image forming apparatus (actually, a control device)1, the photosensitive drums21of the imaging devices20(Y, M, C, K) and the paper transport belt31of the paper transport device3are started to be rotated, and furthermore, each charging device22in the imaging device20charges the image holding surface of each of the photosensitive drums21to have a predetermined polarity and potential. Subsequently, the exposing device23carries out an exposure based on an image signal transmitted from the image processing device (not shown) over the image holding surface of the photosensitive drum21thus charged. Consequently, an electrostatic latent image for each color component having a predetermined potential difference from a charging potential is formed on the image holding surface of each of the photosensitive drums21. Then, the developing device24supplies, from the developing roll24a, a toner charged to have a predetermined polarity and thus develops the electrostatic latent image, thereby forming a toner image. In the developing device24, an inverting phenomenon is carried out. Thus, a toner image having each color (Y, M, C, K) is exclusively formed on the photosensitive drum21of each of the imaging devices (Y, M, C, K).

On the other hand, the recording medium9having a predetermined dimension and type is supplied from the paper feeding device (not shown) toward the paper transport device3and is then adsorbed by an electrostatic function of the adsorbing roll35onto the outer peripheral surface of the rotated paper transport belt31in accordance with a time related to the operation for forming a toner image. When the recording medium9is transported by the paper transport belt31to pass through the transfer position of each of the imaging devices20(Y, M, C, K), thereafter, the toner image having each color (Y, M, C, K) on the photosensitive drum21in the imaging device20is successively transferred to be superposed in order (order of Y, M, C and K) at the recording medium9side upon receipt of a function of an electric field formed by the transferring device25.

Subsequently, the recording medium9having the toner image transferred thereto is peeled from the paper transport belt31and is then transported and introduced toward the fixing device4. In the fixing device4, the recording medium9having the toner image transferred thereto is heated and pressurized in a passage through the pressure contact portion NP between the heating rotor5and the pressurizing rotor6so that the toner of the toner image is molten and fixed to the recording medium9. The recording medium9obtained by ending the fixation is discharged to a paper discharging portion (not shown) and is accommodated therein in the case in which an image is simply formed on either of sides thereof.

Thus, the basic (ordinary) printing operation for a single recording medium is ended. In the case in which an instruction for continuously printing a plurality of sheets is given, furthermore, the serial operation is repeated in the same manner corresponding to the number of the sheets to which the instruction is given.

Next, details of the fixing device4will be described.

In the fixing device4, the heating rotor5taking the roll configuration and the pressurizing rotor6taking the belt configuration are provided in the housing41as shown inFIG. 2. InFIG. 2, the reference numeral42denotes a transporting and introducing plate for introducing the recording medium9to be a fixing target into the pressure contact portion NP, the reference numerals43aand43bdenote a transport guide rib for forming a discharging passage, and the reference numeral44denotes a transport aiding roller.

The heating rotor5taking the roll configuration is mainly constituted by a heating roll51, a heating source52for heating the heating roll51, and a fixing and supporting frame (not shown) for rotatably supporting the heating roll51at both ends thereof.

In the heating roll51, an elastic layer51bconstituted by a material such as a silicone rubber and a mold releasing layer constituted by a material such as a fluororesin (not shown) are formed in this order over a surface of a metallic cylindrical roll base material51awhich has a greater length than a maximum transporting width of the recording medium9to be the fixing target. Moreover, a rotating power is transmitted from a rotating and driving portion disposed on the body side of the image forming apparatus1to a gear55attached to one of ends of the heating roll51so that the heating roll51is rotated and driven at a predetermined speed.

For example, the heating source52is constituted by two halogen lamps disposed in the cylinder of the heating roll51and both ends thereof are supported on the housing41of the fixing device4. In the heating roll51, furthermore, a temperature of the surface thereof is detected by a temperature detector which is not shown and a heating operation of the heating source52is controlled based on information about the detection. Consequently, the surface of the roll is maintained to be heated to a predetermined temperature.

As shown inFIGS. 2 to 5, the pressurizing rotor6taking the belt configuration is mainly constituted by an endless belt61to be rotated in contact with an outer peripheral surface part in a rotating axis direction A of the heating roll51(seeFIG. 3), a pressurizing member62for pushing the endless belt61against the outer peripheral surface part of the heating roll51at an inner peripheral surface side, thereby forming the pressure contact portion NP, and a holding member63disposed in an internal space of the endless belt61and serving to support the pressurizing member62and to rotatably hold the endless belt61. InFIG. 3, the endless belt61and a supporting plate (63D) are not shown.

The endless belt61is a cylindrical belt having a width which is almost equal to the length of the heating roll51. For the endless belt61to be used, a mold releasing layer constituted by a fluororesin is provided on an outer peripheral surface of a belt base material which is formed to take a thin cylindrical shape by a synthetic resin such as polyimide.

As shown inFIGS. 2 to 5, the pressurizing member62is constituted by a head member64taking a slender shape having an almost equal length to the width of the endless belt61, and a pad member65.

The head member64is a hard pressurizing member formed by a material such as a synthetic resin or a metal and is disposed in a position at a downstream side in a passing direction B of the recording medium9in the pressure contact portion NP (a discharging side of the recording medium). In the embodiment, the head member64is formed to take a shape having a protruded portion64apositioned on the downstream side in the passing direction B of the recording medium9and serving to cause the endless belt61to come in pressure contact with the outer peripheral surface of the heating roll51, and a holding portion64bpositioned on an upstream side in the passing direction B of the recording medium9and serving to hold the pad member65. Moreover, the protruded portion64ais provided with a planar portion64a-1for holding a smoothness of the pressure contact portion NP at the side of the holding portion64b, and a curved slant surface64a-2which is slanted in a curving state in such a direction as to separate from the heating roll51in a formation of the pressure contact portion NP at an opposite side to the holding portion64bin the planar portion64a-1(seeFIG. 7).

The pad member65is a soft pressurizing member which is formed by an elastic material such as a rubber material and is disposed in a position at the upstream side in the passing direction B of the recording medium9in the pressure contact portion NP (an introducing side of the recording medium). In the embodiment, the pad member65is formed to take a slender plate-like configuration by using a silicone rubber.

The head member64to be the hard pressurizing member is formed to have a higher hardness than that of the elastic layer51bof the heating roll51. Moreover, the pad member65to be the soft pressurizing member is formed to have a lower hardness than that of the elastic layer51bof the heating roll51and to be elastically deformed. The hardness is represented by a difference in an amount of a deformation in a pressurized part when a pressurization is carried out at a certain pressure.

As shown inFIGS. 2 to 4, the holding member63includes an internal holding member63A having an installing surface portion for installing the pressurizing member62(actually, the head member64) and a belt holding portion for rotatably holding an inner peripheral surface of the endless belt61, a pair of end holding members63B and63C for rotatably holding inner peripheral surface parts of both ends of the endless belt61, and the supporting plate63D for supporting the internal holding member63A and the end holding members63B and63C. The supporting plate63D is held in a state in which a set of two attaching ends63eformed with a protrusion from the both ends are inserted into attaching holes formed on a rock supporting frame71in the pressurizing rotor6which will be described below.

Moreover, the reference numeral68inFIG. 4denotes an oil supplying member attached to a back face of the supporting plate63D to come in contact with the inner peripheral surface of the endless belt61, and is constituted by a felt member impregnated with a mold releasing agent oil, for example. The inner peripheral surface of the endless belt61is coated with the mold releasing agent oil. Moreover, the reference numeral69denotes a film member which is formed by a synthetic resin and is disposed between the inner peripheral surface of the endless belt61and the pressurizing member62, and serves to reduce a friction between the endless belt61and the pressurizing member62.

The pressurizing rotor6is held, through the supporting plate63D of the holding member63, on the rock supporting frame71(an almost central part) to be rocked by using, as a fulcrum, a supporting shaft72disposed in a position at the introducing side of the recording medium. The pressurizing rotor6is pushed to be moved in such a direction as to approach the heating rotor5side by a predetermined pressurizing force F through a pressurizing spring73connected to a free end71aof the rock supporting frame71. Consequently, there is obtained a structure in which the pressurizing member62is pushed against the inner peripheral surface of the endless belt61through the holding member63to apply a predetermined pressure to the pressure contact portion NP.

In the fixing device4, as shown inFIG. 2or5, the pressurizing member62pushes the endless belt61against the heating roll51so that the fixing pressure contact portion NP in which the endless belt61comes in contact with the outer peripheral surface of the heating roll51in a predetermined width (a length in a rotating direction) is formed between the heating roll51and the endless belt61(the pressurizing member62). At this time, the pressure contact portion NP is formed in a state in which both the head portion64and the pad portion65in the pressurizing member62come in contact with the outer peripheral surface of the heating roll51through the endless belt61.

When the heating roll51is rotated as shown inFIG. 2, moreover, the endless belt61is rotated in a direction shown in an arrow to follow the rotation. When the endless belt61is rotated, the mold releasing agent oil is successively coated and supplied in a small amount from the felt member63to the inner peripheral surface of the endless belt61as shown inFIG. 5.

A fixation in the pressure contact portion NP is carried out in the following manner.

More specifically, as shown inFIG. 5or6, when the recording medium9holding an unfixed toner image T to be a fixing target is introduced into the pressure contact portion NP, the pad portion65to be the soft pressurizing member which is disposed on the recording medium introducing side of the pressure contact portion NP first pushes the recording medium9against the heating roll51(through the endless belt61). Subsequently, (the contact portion64aof) the head portion64to be the hard pressurizing member disposed on the recording medium discharging side of the pressure contact portion NP strongly pushes the recording medium9against the heating roll51(through the endless belt61).

The recording medium9holding the unfixed toner image at this time is heated in a state in which it is pushed against the outer peripheral surface of the heating roll51in the heating rotor5by means of the pad portion65and the head portion64in the pressurizing member62of the heating rotor6in the pressure contact portion NP, and furthermore, is transported to pass through the pressure contact portion NP by the rotation of the heating roll51. As a result, when the recording medium9passes through the pressure contact portion NP, the unfixed toner image is heated and pressurized and is thus fixed onto the recording medium9.

In the image forming apparatus1, a specific recording medium9A taking a bag-like configuration such as an envelope can be used as the recording medium9, and the image forming operation can be executed over the specific recording medium9A to form an image.

In some cases in which the image is formed on the specific recording medium9A such as the envelope, however, an unnecessary crease is generated on a rear end at an upstream side in a transporting direction of the recording medium9A after a passage through the fixing device4.

Referring to the generation of the crease, as shown inFIG. 20, for example, when the envelope is to be discharged via the pressure contact portion NP between the heating roll51of the heating rotor5and the pressurizing rotor6taking the belt configuration, it is transported in a curving state in accordance with a curvature of the outer peripheral surface part of the heating roll51passing through the pressure contact portion NP so that a very small difference is made in a moving speed between a surface91and a back face92in an overlapping state of the envelope (a moving speed in an overlapping part on a side provided apart from the heating roll51is higher than a moving speed in an approaching side part). Consequently, it can be supposed that a slack is generated on a surface (for example, the surface91) at a side where the moving speed is relatively low on a rear end side of the envelope introduced into the pressure contact portion NP later and the slack part is finally crushed when passing through the pressure contact portion NP, resulting in the crease.

In the outer peripheral surface part of the heating roll51passing through the pressure contact portion NP in the conventional fixing device4, the elastic layer51bof the heating roll51is dented to be curved in an almost whole region of the pressure contact portion NP by pushing the head member64and the pad member65in the pressurizing member62as shown inFIG. 21. The specific recording medium9A is caused to pass through the pressure contact portion NP in conformity with a curvature of the curved and dented part of the elastic layer51bin the heating roll51in the fixation. There is a tendency that the moving speed difference from the specific recording medium9A is increased when the curvature is increased.

In the fixing device4of the image forming apparatus1, (the protruded portion64aof) the head member64and the pad member65which constitute the pressurizing member62are provided in the following manner as a countermeasure to be taken for suppressing the generation of the crease in the case in which the specific recording medium9A is used as shown inFIG. 6or7.

More specifically, (the protruded portion64aof) the head member64and the pad member65are disposed in a state in which a clearance S is present between both of them (64aand65) in a specific region part (a specific fixing region) E1for causing at least the specific recording medium9A such as an envelope to pass in a fixing set region E in the rotating axis direction A of the heating roll51in the pressure contact portion NP.

The specific fixing region E1corresponds to (a width of) a passing region in the pressure contact portion NP in which the specific recording medium9A such as the envelope is preset to pass in a fixation thereof. In the case in which there is employed a transporting method of carrying out a regulation related to the transporting position of the recording medium9by setting a central position of the fixing set region E of the pressure contact portion NP in the fixing device4as a reference position in a transport, that is, a so-called center registration method, moreover, the specific fixing region E1has normal fixing regions E2and E3on both sides thereof if a feeding width of the specific recording medium9A is not a maximum size in the recording medium9which is applicable (seeFIG. 7). In the normal fixing regions E2and E3, (the protruded portion64aof) the head member64and the pad member65may be disposed in a state in which the clearance S is not present between both of them or is present between both of them.

In the embodiment, in order to dispose them in the state in which the clearance S is present, a member having a gap holding portion66formed in a boundary portion between the protruded portion64aand the holding portion64b(a portion in which a planar erected wall surface64a-3of the protruded portion64acrosses a planar installing surface64b-1of the holding portion64b) is used as the head member64, while a member taking a shape of a linearly rectangular parallelepiped is used as the pad member65as shown inFIGS. 8 to 10.

Furthermore, the gap holding portion66is formed in a state in which a part having a thickness K (a dimension in the passing direction B of the recording medium) corresponding to the gap S to be held on the installing surface64b-1of the pad member holding portion64bin at least the normal fixing regions E2and E3is present. In particular, the gap holding portion66in the example is formed to take such a shape that the thickness K is gradually increased with a shift from the two normal fixing regions E2and E3on both ends toward the specific fixing region E1in the central part (a corresponding part to the fixing region E1has an almost equal thickness). Furthermore, the gap holding member66is formed in a height (a thickness) M which is smaller than a step (a height difference) between the protruded portion64aand the installing surface64b-1of the holding portion64bin the head portion64.

The reason why there is formed the gap holding portion66taking a configuration in which the thickness K is different in the rotating axis direction A of the heating roll51is that the clearance S between (the protruded portion64aof) the head member64and the pad member65is to be reliably maintained in the specific fixing region E1. In the case in which the fixation to the recording medium9A to be a thick paper including an envelope is carried out, moreover, it is necessary to prevent a damage from being caused due to a pressure contact of a tip part in the passing direction of the recording medium9A with the surface of the elastic layer51bof the heating roll51. In addition, the reason why the gap holding portion66is formed in the small height (thickness) M is that a space for a sufficiently elastic deformation is to be maintained when the pad member65forms the pressure contact portion.

The pad member65is disposed in a fixing state, through fixing unit such as bonding, to the installing surface64b-1of the head member64on which the gap holding portion66is formed so that the pressurizing member62is thus finished. Consequently, the pad member65is disposed and held in a state in which the interval K from the protruded portion64aof the head member64(actually, the erected wall surface64a-3) (with respect to the passing direction B of the recording medium) is formed through the presence of the gap holding portion66having a maximum thickness of K (seeFIG. 9).

Referring to the pad member65, moreover, a pressure contact surface portion65aforming the pressure contact portion NP is maintained in a protruding state so as to be present in an approaching direction (a direction shown in an arrow C1) to the heating roll51from the closest surface part to the heating roll51(the plane portion64a-1of the protruded portion64a) in a formation of the pressure contact portion NP in the head member65when the pressurizing member62separates the endless belt61from the heating roll51so that the pressure contact portion NP is not formed before the pressurizing rotor6is assembled or when it is disassembled as shown inFIG. 11A, for example.

In other words, the pad member65is formed and disposed in such a thickness (a height: a dimension from the installing surface64b-1) that the pressure contact surface portion65ais protruded by a predetermined protruding amount a from the plane portion64a-1of the protruded portion64ain the head member64. The designation h inFIG. 11Adenotes a height of the plane portion64a-1of the protruded portion64ain the head member64from the installing surface portion64b-1of the plane portion64a-1. For this reason, the pad member65to be applied is formed in such a dimension that at least a thickness of the pressure contact surface portion65ain a non-pressurizing state is greater than the height h of the plane portion64a-1of the protruded portion64ain the head member64.

In addition, the pad member65is set into a state in which it is elastically deformed and compressed to be present in a separating direction apart from the heating roll51(a direction of an arrow C2) as compared with the surface part of the head member64(the plane portion64a-1of the protruded portion64a) when the pressurizing member62pushes the endless belt61against the heating roll51to form the pressure contact portion NP after the pressurizing rotor6is assembled as shown inFIG. 11B. A one-dotted chain line J inFIG. 11Bindicates a height position of the plane portion64a-1of the protruded portion64a.

In particular, the pad member65is constituted by the soft member to be elastically deformed with a lower hardness than that of the elastic layer51bof the heating roll51as described above. When the pressurizing member62pushes the endless belt61against the heating roll51to form the pressure contact portion NP, therefore, the pad member65is elastically deformed and compressed upon receipt of a reaction force from the elastic layer51b. Furthermore, the protruded portion64aof the head member64cuts into the elastic layer51bat this time. Therefore, the pressure contact surface portion65aof the pad member65is present in a separating direction apart from the heating roll51as compared with the plane portion64a-1to be the surface part of the head member65.

In the fixing device4, in the stage in which the pressurizing rotor6is assembled and disposed to form the pressure contact portion NP between the pressurizing rotor6and the heating roll51, the clearance S is present between the head member64and the pad member65in the pressurizing member62as shown inFIG. 6or11B.

At this time, the pad member65is elastically deformed to be compressed upon receipt of the reaction force of the elastic layer51bof the heating roll51. By the presence of the gap holding portion66, however, the pad member65is rarely deformed with a square section maintained as illustrated inFIG. 6. Actually, the pad member65is deformed in a state in which the upper part of the pressure contact surface portion65ahaving no gap holding portion66is greatly crushed to also enter the space of the clearance S as shown inFIG. 11B. Consequently, the clearance S present between the head member64and the pad member65is caused to be smaller than the thickness K of the gap holding portion66before the assembly or installation.

Also in the case in which the elastic deformation is carried out, there is no problem in that a part (65c) of the pad member65is elastically deformed to come in contact with a part (the wall surface64a-3) of the head member64as illustrated inFIG. 12, and it is sufficient that the elastic deformation is performed to slightly leave the clearance part between of them. In other words, it is preferable that the clearance (S) between the protruded portion64aof the head member64and the pad member65should not be perfectly filled to disappear but should be left also in a state in which the pressurizing rotor6is assembled or a fixing operation.

By the presence of the clearance S, in the pressure contact portion NP, the force of the pad member65to push the endless belt61against the elastic layer51bof the heating roll51is reduced so that the force of the protruded portion64aof the head member64to push the endless belt61and the elastic layer51bis correspondingly increased. As a result, the deformation of the elastic layer51bof the heating roll51in the pressure contact portion NP is relatively increased in a part pushed by the protruded portion64aof the head member64. A part (a range) indicated as Pa inFIG. 6or11serves as a pressure contact portion range formed by a push of the protruded portion64a(mainly, the plane portion64a-1) in the pressure contact portion NP.

Accordingly, the elastic layer51bof the heating roll51in the passage through the pressure contact portion NP takes a shape curved like almost S as a whole by putting together an original part taking a cylindrical shape which is formed in a relationship with the cylindrical roll base member51a(a corresponding part to a pressure contact portion Pb formed between the pad member65and the elastic layer51b) and a deformed (compressed) part having an opposite curvature to a cylindrical shape formed by pushing through the protruded portion64aof the head member64. When the specific recording medium9A such as an envelope passes through the pressure contact portion NP, consequently, a difference in a moving speed in a surface and a back face is reduced so that a crease can be prevented from being generated.

When the force of the pad member65to push the endless belt61against the elastic layer51bof the heating roll51is reduced as described above, moreover, a width of the pressure contact portion NP (a length in the passing direction B of the recording medium9A) is decreased. In the pressure contact portion NP, however, the pad member65is disposed, assembled and attached in a state in which it is protruded from the plane portion64a-1of the protruded portion64ain the head member64(seeFIG. 11B) so that the width of the pressure contact portion NP can be maintained fully and an excellent fixing performance can be obtained.

From the foregoing, according to the fixing device4, it is possible to carry out an excellent fixation having no generation of a crease over the specific recording medium9A such as the envelope. According to the image forming apparatus1using the fixing device4, moreover, it is possible to carry out an excellent image formation without a fixing failure caused by the generation of the crease or a deterioration in the fixing performance.

For reference, in the case in which the head member64and the pad member65are not disposed with the clearance S as shown inFIG. 21, a space for a free deformation is not present on the pad member65side in an elastic deformation to carry out a compression upon receipt of the reaction force of the elastic layer51bof the heating roll51. Therefore, the elastic deformation is brought into a state in which the hardness is increased through the compression to carry out an aggregation close to the pad member65side. As a result, a degree at which the pad member65pushes and deforms the elastic layer51bof the heating roll51is increased so that the range Pb for forming the pressure contact portion NP through the pad member65is enlarged. Thus, the curvature obtained by the deformation of the elastic layer51bin the whole pressure contact portion NP is finally increased as a whole.

FIG. 13shows a result of an evaluation test related to an effect for improving an envelope crease which was carried out by using the fixing device4.

The evaluation test was carried out on the following conditions. There was used the heating roll51obtained by forming the elastic layer51b(a thickness of 0.6 mm) constituted by a silicone rubber (a JIS-A rubber hardness: 40 degrees) on a metallic cylindrical roll base material having an outside diameter of 26 mm For the endless belt61, a belt (a thickness of 60 μm) having an outside diameter of 30 mm and formed of polyimide was used. The head member64to be used was wholly constituted by a molded product formed by a liquid crystal polymer, and had a width of 1 to 2 mm in the plane portion64a-1of the protruded portion64aand had a height h of 3 mm of the protruded portion64afrom the installing surface64b-1of the holding portion64b. The pad member64which was used is formed by a silicone rubber (a hardness of Ascar C (9.8 N): 24 degrees) and takes a shape of a square bar having a width (a length in the passing direction B) of 5 mm and a thickness of 3.5 to 4.2 mm.

In the fixing device4, the endless belt61is pushed against the heating roll51by means of the pressurizing member62including the head (Head) member64and the pad (PAD) member65in a state in which a load of 170 N is applied, and the pressure contact portion (Nip) NP is thus formed. The heating roll51is rotated at a rotating speed of 90 mm/sec and is heated to hold a surface temperature to be 180° C.

For the specific recording medium9A, two types of envelopes (manufactured by Columbia, form: Com-10, model number: CO125, CO131) were used. Both feeding widths in a passage of the envelopes to pass through the pressure contact portion NP are 105 mm “Surface” and “Back” inFIG. 12indicate a surface and a back face in each of the two types of envelopes.

Moreover, the center registration method is employed in the fixing device4. Therefore, a region portion of 120 mm is maintained as the specific fixing region E1of the pressure contact portion NP in a central part of the fixing set region E (a total width of 220 mm). The pad member65is disposed in each clearance (K:FIG. 9) and a protruding amount (α:FIG. 9) shown inFIG. 13on the installing surface64b-1of the holding portion64bin the head member64, and the pressure contact portion NP is then formed. The pressure contact portion NP has a range from a point where the pad member65is started to come in contact with the outer peripheral surface of the heating roll51through the endless belt61to a point where the head member64is released from the contact of the outer peripheral surface of the heating roll51through the endless belt61. In the test, a width of each pressure contact portion NP (an Nip width) was obtained by measuring a width of an uneven fixing part in a solid image (a length in the passing direction B) when introducing and stopping the recording medium9having the solid image preformed thereon in the pressure contact portion NP of the fixing device4.

For the fixing device4having each structure, the fixation was carried out through two types of envelopes, and the envelopes subjected to the fixation were observed to examine a situation in which a crease was generated at a rear end to be an upstream side in the passing direction B. In the test, character information about a destination such as an address was formed as a toner image on the envelope to fix an unfixed toner image. The situation of the generation of the crease was evaluated based on the following reference.Grade 0: A crease was not generated.Grade 1: A crease was generated within a range of an inside of 10 mm from a rear end.Grade 2: A crease was generated within a range of an inside of 20 mm from the rear end.Grade 3: A crease was generated within a range exceeding 20 mm from the rear end.Grade 4: Such a crease as to change a shape of the rear end was generated.

Referring to the result of the test, moreover,FIG. 14Ashows a result of the width of the pressure contact portion NP with respect to the protruding amount of the pad member in a graph for each clearance. In this case, 5 mm or more is required for maintaining an excellent fixing performance. Furthermore,FIG. 14Bshows a result of an evaluation grade of an envelope crease with respect to the protruding amount of the pad member in a graph for each clearance. In this case, an allowable grade which is fit for a practical use is Grade 1 or less.

Another Embodiment

In the embodiment, the pad member65to be used takes the shape of the square bar having a square section. In addition, it is also possible to use a member taking such a shape as to have a dimension W in a passing direction B of a recording medium increased gradually apart from a heating roll51as shown inFIG. 15, for example.

A pad member65illustrated inFIG. 15takes a trapezoidal shape in which a width W2of an installing surface (a bottom face)65bis greater than a width W1of a pressure contact surface portion65afor both of upstream and downstream sides in the passing direction B. In the case in which the pad member65taking the shape is used, it is hard to generate a sequentially permanent strain by an influence of a high temperature environment under a pressurization in a fixation in a pressure contact portion NP of the pad member65. Thus, it is possible to stably maintain a state of the pressure contact portion NP over a long period of time. As a result, it is possible to obtain an effect for suppressing a generation of a crease in the fixation of a specific recording medium9A over a long period of time. In this case, the pad member65may take such a trapezoidal shape that the width W2of the installing surface65bis greater than the width W1of the pressure contact surface portion65atoward the upstream or downstream side in the passing direction B.

Although there has been described the case in which the two planes (64a-1,64a-3) are formed to cross each other at an almost right angle as a corner portion64eto be the upstream side in the passing direction B of the recording medium in the protruded portion64aof the head member64in the embodiment (seeFIGS. 7 and 10), moreover, it is also possible to form the corner portion as a slant surface67which approaches the heating roll51with a shift toward the downstream side in the passing direction B as shown inFIG. 16or17. The slant surface67corresponds to a surface which is referred to as a so-called taper surface or round surface, and a planar shape thereof may be a flat surface or a curved surface.

Referring to a pressure distribution in the pressure contact portion NP in a fixing device4, as shown inFIG. 18, a pressure in a pressure contact portion Pa formed by pushing the protruded portion64aof the head member64to be a hard pressurizing member is the highest and a pressure in a pressure contact portion Pb formed by pushing the pad member65to be a soft pressurizing member is relatively lower than that in the pressure contact portion Pa. However, a pushing force of a pressurizing member62does not directly reach a pressure in a pressure contact portion Pc corresponding to a clearance S present between the protruded portion64aof the head member64and the pad member65. Therefore, there is set a low pressure state in which the pressure approximates to zero. In the case in which a fixation to a recording medium9to be a thick paper including an envelope is carried out, therefore, a tip part of the thick paper is maintained to be strongly pushed against an elastic layer51bside of the heating roll51when the tip part of the thick paper is introduced into the pressure contact portion NP and enters and passes through a high pressure region of the pressure contact portion Pa at a downstream side from a low pressure region of the pressure contact portion Pc. Therefore, a great load is instantaneously applied to the elastic layer51bportion so that a surface of the elastic layer51bis damaged. In some cases, consequently, a picture quality failure is caused by the damage.

On the other hand, in the case in which a corner portion (64e) to be an inlet side of the protruded portion64ain the head member64is formed as the slant surface67, the pressure distribution in the pressure contact portion Pa is slowly raised by the presence of the slant surface67. When the tip part of the thick paper enters and passes through the pressure contact portion Pa, consequently, a pressure for a push toward the elastic layer51bside of the heating roll51is reduced so that the load to be applied to the elastic layer51bportion is also reduced. As a result, there is reduced a damage which is caused by the pressure contact of the tip part of the thick paper in the elastic layer51bof the heating roll51. Thus, it is possible to avoid the picture quality failure caused by the damage.

Moreover, the slant surface67is formed in a state in which a length L in the passing direction B of the recording medium is greater in (a length L1of) a central part than (lengths L2and L3of) an end in a rotating axis direction A of the heating roll51in a specific fixing region E1as shown inFIG. 16. It is preferable that the length L of the slant surface67should be set to be further smaller than that in each of normal fixing regions E2and E3which are present on an outside of the specific fixing region E1.

In this case, it is also possible to prevent a so-called raindrop-like picture quality failure from being caused in an execution of a fixation using a coating paper or an OHP sheet as the recording medium9. In other words, it is also possible to prevent the raindrop-like picture quality failure from being caused, and at the same time, to prevent a crease from being generated in the execution of the fixation to the recording medium9such as the envelope and to prevent a defect such as a damage from being caused in the execution of the fixation using the envelope or the thick paper. The raindrop-like picture quality failure is caused due to a flow of a toner heated and molten in the pressure contact portion NP in a place (a valley) having a low pressure if the pressure contact portion NP includes the place having a low pressure between the head member64and the pad member65as illustrated inFIG. 18.

In the case in which the slant surface67is formed on the head member64, the part having a low pressure is enlarged by the presence of the slant surface67. For this reason, the raindrop-like picture quality failure is apt to be caused. In particular, the raindrop-like picture quality failure is apt to be caused in an end region in the rotating axis direction of the heating roll. By forming the slant surface67to be long in the central part of the specific fixing region E1and to be relatively short at the ends, however, it is possible to suppress the raindrop-like picture quality failure which is apt to be caused at the ends.

Although there has been described the example of the structure in which the gap holding portion66is formed in the head member64in order to cause the clearance S to be present between the protruded portion64aof the head member64and the pad member65in the embodiment, furthermore, the invention is not restricted to the case in which the means is employed. In addition, for example, it is also possible to employ a structure in which the pad member65is attached in a fixing state to a supporting plate75having a higher hardness than that of the pad member65and the whole supporting plate75is provided on an installing surface64b-1of a holding portion64bin the head member64as shown inFIG. 16or17.

In this case, the pad member65is disposed in a fixing state to a position shifted inward from one end75aof the supporting plate75by a distance G corresponding to the clearance S present between the pad member65and the protruded portion64aof the head member64. For the supporting plate75, it is possible to use a hard plate formed by a synthetic resin or a metal plate. The supporting plate75does not need to take a shape of a flat plate if a gap holding function can be exhibited. With the structure, it is possible to dispose the head member64and the pad member65accurately and easily in a state in which the clearance S is present. Thus, it is also possible to reliably obtain an effect for preventing a crease of the specific recording medium9A from being generated.

In the fixing device4, moreover, it is also possible to use, as the pad member65, a porous elastic member (for example, a sponge or a porous rubber) which is easily deformed elastically at a certain pressure and to dispose a porous pad member65B on the head member64in a state in which the clearance S is present. However, the porous pad member65B may be disposed on the head member64in a state in which the clearance S is not present as illustrated inFIG. 19. InFIG. 19, the reference numeral65cdenotes a void part to be a hole.

In this case, the pad member65B constituted by the porous elastic member is freely deformed elastically without a particular restriction upon receipt of a reaction force of the elastic layer51bof the heating roll51when the pressure contact portion NP is formed as a part of the pressurizing member62. In the pressure contact portion NP, consequently, a force of the pad member65B to push an endless belt61against the elastic layer51bof the heating roll51is reduced and a force of the protruded portion64aof the head member64to push the endless belt61and the elastic layer51bis increased correspondingly. As a result, the deformation of the elastic layer51bof the heating roll51in the pressure contact portion NP is relatively increased in a part pushed by the protruded portion64aof the head member64.

Accordingly, the elastic layer51bof the heating roll51in a passage through the pressure contact portion NP takes an almost S-like curved shape as a whole by putting together an original cylindrical part formed in a relationship with a cylindrical roll base member51aand a deformed part having an opposite curvature to the cylindrical shape formed by a push through the protruded portion64aof the head member64in almost the same manner as in the case according to the embodiment. Also when the specific recording medium9A such as the envelope passes through the pressure contact portion NP applying the pad member65B, consequently, a difference in a moving speed on a surface and a back face can be reduced so that a crease can be prevented from being generated.

Moreover, it is also possible to employ a structure in which the clearance S is present by forming a gap holding portion in the pad member65. Furthermore, it is also possible to employ a structure in which a concave portion (a shape of a dent) to be protruded toward the downstream side in the passing direction B in the rotating axis direction A is formed on a boundary wall surface portion64a-3of the protruded portion64ain the head member64and a gap part of the concave portion is present as the clearance S.

In addition, the specific fixing region E1can also be set to be the whole fixing set region E. For the specific recording medium9A, it is also possible to apply recording media of types other than the envelope if the generation of the crease can be suppressed.

Furthermore, an imaging device for forming a monochrome toner image may be applied to the imaging device2in the image forming apparatus1. The imaging device2does not need to employ a transferring method.