Belt member incorporated in image forming apparatus

A belt member is formed with a seam portion by overlapping and adhering both longitudinal end portions thereof to constitute an endless belt stretched and circulated by a rotative, first stretching member and a second stretching member. A length of the seam portion is no less than a length between a first point at which the endless belt is separated from one of the first stretching member and the second stretching member and a second point at which the endless belt is brought into contact with the other one of the first stretching member and the second stretching member.

BACKGROUND OF THE INVENTION

The present invention relates to a belt member formed with a seam portion and incorporated in an image forming apparatus.

In an image forming apparatus, there is used a belt-shaped film such as a photosensitive film and an image fixing film. For example, Japanese Patent Publication No. 8-187773A discloses an image fixing film formed with an overlapped portion at which longitudinal ends of the film are overlapped. It is disclosed to form a seamed endless belt by bonding the overlapped portion (hereinafter, referred to as seam portion).

The seam portion is formed by a length shorter than a total length of the endless belt. When the seamed endless belt is supported between stretching members and driven to circulate, since the length of the seam portion is extremely shorter than a distance between the stretching members, there poses a problem that when the seamed endless belt is repeatedly used, a damage is caused such that the seam portion is exfoliated or the belt is cut.

FIGS. 31 and 32are sectional views showing a constitution of a film70described in the above publication. InFIG. 31, numerals71and72designate longitudinal ends of a substrate, and a seam portion72is formed by adhering the overlapped ends. A thickness of the substrate at the seam portion becomes Wa to bring about a stepped difference.

There is a case in which the substrate having a stepped difference cannot be run smoothly or a case in which the substrate causes to produce damage. Therefore, as shown byFIG. 32, a flattened portion73is formed at the seam portion by pressing to flatten the stepped difference by applying heat and pressure.

According to the above constitution, only one sheet of the substrate is constituted except the seam portion and therefore, there is a case in which the strength is deficient when the film is used as a belt member of a photosensitive film, an image fixing film or the like in an image forming apparatus. Therefore, a problem of destructing the belt member is posed. Further, there poses a problem that the large stepped difference of the belt member in running impinges on other elements of the apparatus, so that the belt member is damaged.

Further, as shown byFIG. 32, since the seam portion is pressed to flatten to constitute the flattened portion, a density of the flattened portion becomes approximately twice as much as that of other portion. Therefore, when such a substrate is used as an image fixing film, transfer of temperature to a recording medium is deteriorated at the portion to thereby pose a problem that a failure in fixing is brought about.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a belt member capable of preventing a seam portion from being damaged.

It is also an object of the invention to provide a belt member capable of increasing a strength of the seam portion and preventing a fixing failure from being occurred.

It is also an object of the invention to provide a fixing device or an image forming apparatus incorporating such a belt member.

In order to achieve the above objects, according to the invention, there is provided a belt member, comprising a seam portion formed by overlapping and adhering both longitudinal end portions thereof to constitute an endless belt stretched and circulated by a rotative, first stretching member and a second stretching member,

wherein a length of the seam portion is no less than a length between a first point at which the endless belt is separated from one of the first stretching member and the second stretching member and a second point at which the endless belt is brought into contact with the other one of the first stretching member and the second stretching member.

In this configuration, shear force exerting to the seam portion can be reduced so that the service life of the endless belt is prolonged.

According to the invention, there is also provided an image forming apparatus, comprising:

the above belt member;

a photosensitive member, operable to support a toner image thereon; and

a transfer member, operable to transfer the toner image from the photosensitive member onto a recording medium transported by the belt member.

In this case, it is possible to prevent the seam portion of the endless belt used in the transfer operation from being damaged.

According to the invention, there is also provided an image forming apparatus, comprising:

the above belt member;

a photosensitive member, operable to support a toner image thereon;

a primary transfer device, operable to transfer a toner image from the photosensitive member onto the belt member; and

a secondary transfer device, operable to transfer the toner image from the belt member onto a recoding medium.

In this case, it is possible to prevent the seam portion of the endless belt used in the intermediate transfer operation from being damaged.

Here, it is preferable that the image forming apparatus further comprises a third stretching member and fourth stretching member which are arranged such that a circulating path of the belt member is made trapezoidal. In this case, the endless belt can be circulated smoothly.

According to the invention, there is also provided an image forming apparatus, comprising:

the above belt member;

a heat generator, provided with the first stretching member; and

a fixing member, arranged so as to abut against the first stretching member through the belt member, so that a toner image formed on a recording medium is fixed thereon when the recording medium is placed at a nip portion between the fixing member and the belt member.

In this case, it is possible to prevent the seam portion of the endless belt used in the fixing operation from being damaged.

Here, it is preferable that the image forming apparatus further comprises a third stretching member and fourth stretching member which are arranged such that a circulating path of the belt member is made trapezoidal. In this case, the endless belt can be circulated smoothly.

It is also preferable that the image forming apparatus further comprises:

a photo sensitive member, operable to support a toner image thereon; and

a transfer member, operable to transfer the toner image from the photosensitive member onto a recording medium which is to be transported to the nip position.

Alternatively, it is preferable that the image forming apparatus further comprises:

a photosensitive member, operable to support a toner image thereon;

an intermediate transfer member;

a primary transfer device, operable to transfer a toner image from the photosensitive member onto the intermediate transfer member; and

a secondary transfer device, operable to transfer the toner image from the intermediate transfer member onto a recoding medium which is to be transported to the nip portion.

According to the invention, there is also provided an image forming apparatus, comprising:

the above belt member; and

a fixing member, provided with a heat generator and arranged so as to abut against the first stretching member through the belt member, so that a toner image formed on a recording medium is fixed thereon when the recording medium is placed at a nip portion between the fixing member and the belt member.

Here, it is preferable that the image forming apparatus further comprises:

a photo sensitive member, operable to support a toner image thereon; and

a transfer member, operable to transfer the toner image from the photosensitive member onto a recording medium which is to be transported to the nip position.

Alternatively, it is preferable that the image forming apparatus further comprises:

a photosensitive member, operable to support a toner image thereon;

an intermediate transfer member;

a primary transfer device, operable to transfer a toner image from the photosensitive member onto the intermediate transfer member; and

a secondary transfer device, operable to transfer the toner image from the intermediate transfer member onto a recoding medium which is to be transported to the nip portion.

It is also preferable that the second stretching member has a semiannular shape. In this case, the product cost can be reduced.

Preferably, the belt member is wound by a plurality of turns so that the length of the seam portion is made no less than a circumference of the endless belt.

In this configuration, sufficient strength can be assigned to the belt member while reducing the stepped difference at the seam portion. Therefore, it is possible to prevent the endless belt being damaged.

Here, it is preferable that the belt member is formed with a stepped portion through which both longitudinal ends of the belt member oppose to each other in a circumferential direction of the endless belt.

In this case, the thickness of the endless belt can be entirely uniformed. The stepped portion can be formed by applying heat and pressure.

According to the invention, there is also provided an image forming apparatus, comprising:

a rotative, first stretching member;

a second stretching member;

a belt member, comprising a seam portion formed by overlapping and adhering both longitudinal end portions thereof to constitute an endless belt stretched and circulated by the first stretching member and the second stretching member;

a fixing member, provided with a heat generator and arranged so as to abut against the first stretching member and the second stretching member through the belt member, so that a toner image formed on a recording medium is fixed thereon when the recording medium is placed at a nip portion between the fixing member and the belt member;

wherein a length of the seam portion is no less than a length between a first point at which the fixing member is abutted against the first stretching member through the belt member and a second point at which the fixing member is abutted against the second stretching member through the belt member.

In this configuration, shear force exerting to the seam portion can be reduced so that the service life of the endless belt is prolonged.

Here, it is preferable that the second stretching member has a semiannular shape. In this case, the product cost can be reduced.

It is also preferable that the image forming apparatus further comprises:

a photo sensitive member, operable to support a toner image thereon; and

a transfer member, operable to transfer the toner image from the photosensitive member onto a recording medium which is to be transported to the nip position.

Alternatively, it is preferable that the image forming apparatus further comprises:

a photosensitive member, operable to support a toner image thereon;

an intermediate transfer member;

a primary transfer device, operable to transfer a toner image from the photosensitive member onto the intermediate transfer member; and

a secondary transfer device, operable to transfer the toner image from the intermediate transfer member onto a recoding medium which is to be transported to the nip portion.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the invention will be described below with reference to the accompanying drawings.

FIG. 1shows a configuration in which an endless belt1is stretched between a drive stretching member2and a stretching member3and circulated by a drive force of the drive stretching member2.

A point P1at which the endless belt1starts contacting with the drive stretching member2is exerted with a stretching force F1of a total of a tension force F5(N) and a force by driving to rotate the driving stretch member2by a torque T (N·m). Here, T1=(T/R1)+F5(N). Incidentally, notation R1designates a diameter of moving the drive stretching member2. Further, a point P2at which the endless belt1starts separating from the stretching member3is exerted with a reaction force F2. Here, in order to move to rotate the endless belt at equal velocity, F2=F1=(T/R2)+F6(N). Incidentally, notation R2designates a diameter of moving the stretching member3and notation F6designates a tension reaction force.

Further, the reaction force F2is constituted by a friction force exerted to the endless belt1by the stretching member3, or a friction torque of the stretching member3, or an axially supporting force or a total of these. Between the point P1and the point P2, the endless belt1is stretched by the stretching force F1and the reaction force F2in opposed directions. Meanwhile, at a point P3or a point P4of portions of the endless belt1in contact with the stretching member3or the drive stretching member2, the stretching force F1and a friction force F3which the endless belt1receives from the stretching member3(hereinafter, the friction force generally designates a resistance force with respect to a force including static friction, dynamic friction, a friction torque of the belt1and the drive stretching member2or the stretching member3) are canceled by each other. Further, the stretching force F1and a friction force F4received from the driving stretch member2are canceled by each other and therefore, a force of stretching the endless belt is smaller than the stretching force F1.

Similarly, at a point P5at which the endless belt1starts contacting with the stretching member3, the stretching force F1and the friction force F3are canceled by each other. Further, also at the point P6at which the endless belt1starts separating from the drive stretching member2, the stretching force F1and the friction force F4are canceled by each other. Therefore, a force of stretching the endless belt1is smaller than the stretching force F1. Therefore, between the point P5and the point P6, the force of pulling the endless belt1is smaller than the stretching force F1.

Next, an explanation will be given of equilibrium of force of a seamed endless belt. InFIG. 2, numeral4designates a film, numeral5designates a seam portion overlapping both longitudinal end portions of the film4so that the endless belt is formed by the film4. A film on an upper side of the seam portion5is designated by notation4aand a film on a lower side thereof is designated by notation4b.

FIG. 3is a schematic view showing forces exerted to the seam portion5when the seam portion5ofFIG. 2is disposed between the point P1of the drive stretching member2and the point P2of the stretching member3. InFIG. 3, the seam portion5of the endless belt is coated with an adhering material to form an adhering layer6. The adhering layer6is exerted with a shear force γ1by the stretching force F1and a shear force γ2by the reaction force F2.

Here, when a length of the seam portion5is designated by notation L(m) and a width of the belt (width of adhering layer) is designated by notation W(m), the following equation of γ1=F1/(L·W)=F2/(L·W)=γ2(N/m) is established. It is known from the this equation that the shear force γ1(γ2) is reduced in an inverse proportion to the length L of the seam portion.

FIG. 4is a diagram showing a relationship between the length L of the seam portion5and the shear force γ1(γ2). As shown, the shear force γ1(γ2) is reduced in the inverse proportion of the length L of the seam portion. Further, at L>Lh, the shear force γ1(γ2) becomes constant.

It appears that the shear force γ1(γ2) exerted to the adhering layer6can be minimized by making the length L of the seam portion5equal to or larger than a distance Lh between the point P1and the point P2. Further, when the distance between the point P1and the point P2ofFIG. 1is set to Lh (m), that is, when L=Lh, the stretching force F1and the reaction force F2are reduced by the friction forces F4and F3of the drive stretching member2and the stretching member3. Therefore, also the shear force γ1(γ2) is reduced.

Next, an explanation will be given of forces exerted to the seam portion5when the seam portion5is disposed between the point P1and the point P2and is brought into contact with the drive stretching member2. InFIG. 5, a portion A is a portion at which the seam portion5is not brought into contact with the drive stretching member2and a portion B is a portion at which the seam portion5is brought into contact with the driving stretch member2. At the portion A, the adhering layer6is exerted with a shear force γ3by a stretching force F7and a shear force γ5by a reaction force F9. At the portion B, the adhering layer6is exerted with a shear force γ4by a stretching force F8and a shear force γ6by a reaction force F10.

Further, a relationship between the stretching forces F7and F8becomes F8<F7by the friction force received from the drive stretching member2. Further, a relationship between the reaction forces F9and F10becomes F10<F9. Thereby, relationships among the stretching force and the reaction force and the shear forces inFIG. 3become F7+F8<F1, γ3+γ4<γ1, F9+F10<F2and γ5+γ6<γ2. Therefore, when a portion of the seam portion5is brought into contact with the drive stretching member2, the shear forces exerted to a total of the adhering layer6becomes smaller than γ1and γ2ofFIG. 3.

Therefore, the shear force γ1(γ2) exerted to the adhering layer6can be minimized by making the length L of the seam portion5equal to or larger than the distance Lh between the point P1and the point P2. Further, the shear force γ1(γ2) exerted to the adhering layer6can be minimized whenever at least a portion of the seam portion5is disposed between the point P1and the point P2.

Next, an explanation will be given of forces exerting to the seam portion5when the seam portion is disposed between the point P1and the point P2and is brought into contact with the stretching member3. InFIG. 6, at the belt member4bbelow the adhering layer6, a stretching force F11and a reaction force F12are canceled by each other and therefore, a shear force applied to the adhering layer6becomes null. Further, the stretching force F11is equal to the stretching force F1ofFIG. 1and the reaction force F12is equal to the reaction force F2ofFIG. 1.

Further, an explanation will be given of a case in which a layer upward from and a layer downward from the seam portion5of the endless belt are reversed inFIGS. 5 and 6. When a way of winding the film4is reverse to that of the example ofFIG. 2, the drive stretching member2inFIG. 5is replaced with the stretching member3. Further, by replacing the stretching member3inFIG. 6with the drive stretching member2, forces exerted to the seam portion5of the endless belt can be explained similar toFIGS. 5 and 6, as mentioned above.

In this way, the endless belt according to the invention can minimize the shear force exerted to the seam portion5as shown byFIG. 4, so that the lifetime thereof can be prolonged. The endless belt can be used as a belt member for a photosensitive film, an image fixing film or the like in an image forming apparatus as mentioned later.

As a first embodiment, an explanation will be given of an example of applying a seamed endless belt according to the invention as a photosensitive film for an image carrier.

i) A substrate is constituted by a film of polyester resin having a thickness of 50 μm, a width of 340 mm and a length of 234 mm. Otherwise, polycarbonate or the like can be used as the substrate.

ii) A binder resin of polymethylmetacrylate is dissolved to toluene. Next, a conductive coating prepared by dispersing carbon black thereto is coated on a surface of the film (extrusion coater method) and dried to form a conductive layer having a thickness of 25 μm. Other than forming the conductive layer as described above, the conductive layer may be formed by vapor-depositing aluminum by 1000 Å.

iii) Copolymer nylon (nylon 6 or nylon 66 or nylon 12) dissolved in butanol is coated on the conductive layer formed as described above (extrusion coater method) and dried to form an under coating layer having a thickness of 1 μm.

iv) Dyan blue (kind of azo pigment) as a charge generating substance and polycarbonate resin as a binder resin are dissolved in n-butylamine to thereby prepare a coating solution of a charge generating layer. As a charge generating substance, sudan red, disazo pigment, quinone pigment, phthalocyanine pigment, pyrylium salt, or azulenium salt can be used. Further, as a binder resin, polysteyrene, polymethacrylate ester, polyester, or cellulose ester can be used. Further, as a solvent, diethylamine, ethylenediamine or acetone can be used.

v) The above-described coating solution is coated on the under coating layer (extrusion coater method) and dried to thereby form a charge generating layer having a thickness of 0.81 μm.

vi) Hydrazone compound as a charge transporting substance and polycarbonate resin as a binder resin are dissolved in n-butylamine to thereby prepare a coating solution of a charge transporting layer. As a charge coating substance, a compound including a polycyclic aromatic compound of anthracene, pyrene or the like at a principal chain or a side chain thereof, or a compound having a skeleton of nitrogen including cycle compound of indole, carbazole or the like can be used. Further, as a binder resin, polystyrene, polymethacrylate ester, polyester, cellulose ester can be used. Further, as a solvent, diethylamine, ethylenediamine, or acetone can be used.

vii) The above-described coating solution is coated on the charge generating layer (extrusion coater method) and dried to thereby form a charge transporting layer having a thickness of 17 μm.

FIG. 7is a schematic sectional view of a photosensitive film7formed by the above-described steps of i) through vii). In this figure, notation7adesignates a substrate comprised of a polyester resin film, notation7bdesignates a conductive layer, notation7cdesignates an under coating layer, notation7ddesignates a charge generating layer and notation7edesignates a charge transporting layer. Both longitudinal ends of the photosensitive film7formed in this way are overlapped so as to form an overlapped portion. The overlapped portion is welded by ultrasonic welding to thereby form a seam portion.

When the ultrasonic welding is performed, as shown inFIG. 8, the charge transporting layer7eis arranged to dispose on an outer side of the photosensitive film7.

The overlapped portion of the photosensitive film7is held at a welding table9of an ultrasonic welder and a horn8is brought into contact thereon by pressing force of 50 kgf/cm2. Further, the horn8is moved at a velocity of 30 mm/min in a direction of an arrow T while applying an ultrasonic wave having a frequency of 20 kHz and an amplitude of 20 μm. As a result, the seam portion5is formed by welding the overlapped portion by ultrasonic welding. A welding method by heat and pressure or an adhering method by an adhering agent can be used instead of using the ultrasonic welder. A width Z on the seam portion is 55 mm and a diameter of the formed seamed photosensitive film is φ60 mm.

Next, an explanation will be given of an example of incorporating a seamed endless belt comprising a photosensitive film in an image forming apparatus. InFIG. 9, the drive stretching member2and the stretching member3are respectively constituted by pipes and the photosensitive film1(hereinafter, referred to as a photosensitive belt member) is stretched between the drive stretching member2and the driving member3. The tension force between the drive stretching member2and the stretching member3in this case is 26 N.

Further, as a constitution of the pipe used for the drive stretching member2and the stretching member3, the pipe is made of aluminum having an outer diameter of φ25 mm and a wall thickness of 1.6 mm and a length of 372 mm and coated with urethane having a thickness of 50 μm at a surface thereof. A distance between centers of the pipes is 55 mm. A photosensitive member unit having such a constitution is incorporated in an image forming apparatus10.

InFIG. 9, numeral11designates a developing unit which is provided with a developing roller11a, a toner supply roller11b, a toner control blade11c, and a toner agitator11d. Numeral12designates light ray irradiated from an exposure unit, numeral13designates a charging unit, numeral14designates light ray irradiated from a discharging unit, numeral15designates a cleaner unit, and numeral16designates a fixing unit. The fixing unit16is provided with a heating roller16ahaving a heater H at inside thereof and a pressing roller16b. Numeral18designates a transferring unit which is constituted by the drive stretching member2and a transferring roller18a. Numeral17designates recording paper which is carried in a direction of an arrow Q.

Next, an explanation will be given of a procedure of forming an image by the image forming apparatus10.

(1) The drive stretching member2starts driving the photosensitive belt member1to circulate in a direction of an arrow R.

(2) The photosensitive belt member1is charged to −600V by the charging unit13.

(3) An electrostatic latent image is formed on the photosensitive belt member1by the light ray12from the exposure unit. Further, in the processing, charge at an exposed portion is nullified and charge at an unexposed portion remains.

(4) The toner is charged negatively by friction by the developing unit11to develop the electrostatic latent image formed on the photosensitive belt member1. In the processing, the charge nullified portion of the exposed portion is filled by the charged toner to develop.

(5) The recording paper17is carried in the arrow Q direction and advances between the transferring roller18aof the transferring unit18and the photosensitive belt member1.

(6) The transferring unit18is applied with transferring bias voltage making current of +20 μA flow and a toner developed image is transferred from above the photosensitive member1to the recording paper17(recording medium).

(7) The recording paper17transferred with the toner image is carried to the fixing unit16. At the fixing unit16, the toner image on the recording paper17is melted to fix by operation of heat and pressure.

(8) The toner, paper powder or the like remaining on the photosensitive member1which has passed through the transferring unit18is scraped off by the cleaner unit15.

(9) Further, the light ray14is irradiated from the discharging unit and the remaining electrostatic latent image on the photosensitive belt member1is nullified.

(10) The operation returns to the processing of (2) in the case of continuous printing.

An example of a condition of forming the image is as follows. Drive torque of the drive stretching member2is 0.076 (N·m). Therefore, the stretching force F1ofFIG. 1becomes (0.076/0.0125)+26=32 N. The distance between the point P1and the point P2is 55 mm. The conductive layer of the photosensitive belt member1is connected to the ground. As a method therefor, the conducive layer is exposed at an end portion of the belt and is brought into contact with a conductive blush terminal connected to the ground. With regard to rotational speed of the photosensitive belt member, surface speed is 215 mm/sec and paper passing speed is 40 ppm in passing paper of A4 in the transverse direction.

Next, an explanation will be given of a relationship between a length and a durability of the seam portion. Durability is evaluated by changing the length of the seam portion5and carrying out continuous printing by the image forming apparatus shown inFIG. 9. A print image at this occasion is a character image of A4 size. In evaluating the durability, the image forming apparatus is stopped at each continuous printing of 500 sheets, a lid of the apparatus is opened and it is observed with eyes whether there is a damage of exfoliation, float-up, crack, break or the like at the seam portion5of the photosensitive belt member1. Further, a total number of sheets of passing paper at a time point of bringing about the damage is defined as a life printing sheet number.

Table 1 shows the length of the seam portion and a result of evaluating the life printing sheet number.

As shown in Table 1, although there are more or less measurement dispersion, by making the seam length longer than the distance 55 mm between the point P1and the point P2ofFIG. 9, the shear force applied on the seam portion can be reduced. Further, the life printing sheet number can be increased.

TABLE 1length of seam portion (mm)life printing sheet number757950065805005581000454050035310002522500151250010belt is cut immediately5belt is cut immediately

In a case where at least one of the charging unit13, the developing unit11, the transferring unit18, and the cleaner unit15in the image forming apparatus ofFIG. 9is a contact-type device, the force of driving to circulate the photosensitive belt member1needs to be higher than that in a non-contact type device. Therefore, the shear force applied to the seam portion is also increased and a degree of breaking the photosensitive belt member1is also increased. However, since the shear force can be reduced by the constitution of the invention, the effect of preventing the photosensitive belt member1from being broken is further enhanced.

Here, the stretching member3may be arranged to be opposed to the transferring member18ainFIG. 9.

As a second embodiment, an explanation will be given of an example of applying a seamed endless belt according to the invention as an intermediate transferring member in an image forming apparatus.

i) As a substrate, a conducive resin film having a thickness of 300 μm, a width of 340 mm and a length of 975 mm is used. The film is dispersed with 20 wt % of carbon black powder as a conductive agent in polyurethane resin.

iii) Otherwise, as a conductive agent, perchlorates, or zinc oxide, tin oxide, antimony oxide, titanium oxide, respectively made conductive by doping antimony, indium or the like or metal particles or metal fibers of Cu, Al, Ni, stainless steel, or iron or carbon fiber can be used.

iv) The overlapped portion is formed by overlapping both longitudinal ends of the film (seeFIG. 2) and the overlapped portion is adhered by an adhering agent (1521; Three Bond Co., Ltd.). Otherwise, the overlapped portion may be melted to adhere by heat and pressure.

v) A length of a seam portion is 347 mm and a diameter of a seamed endless intermediate transferring belt is φ200 mm.

The intermediate transferring member according to this embodiment is stretched by 4 pieces of pipes to constitute an intermediate transferring unit as shown inFIG. 10. As a constitution of the pipe, the pipe is made of aluminum having an outer diameter of φ30 mm, a wall thickness of 1.6 mm and a length of 372 mm and surface thereof is coated with urethane having a thickness of 50 μm.

InFIG. 10, notations21athrough21care first through third stretching members and numeral22designates a drive stretching member. Since a transporting path is formed in a trapezoidal shape by 3 pieces of the stretching members and the drive stretching member in this way, the endless belt can stably be run. The intermediate transferring belt23is stretched among the first through the third stretching members21athrough21cand the drive stretching member22. A length of the intermediate transferring belt23is selected such that La is 180 mm, Lb is 224 mm and Lc is 65 mm.

FIG. 11is an explanatory view showing equilibrium of forces exerting to the transferring unit shown inFIG. 10. When the drive stretching member22is driven to rotate, the intermediate transferring belt23is circulated in an arrow S direction. At this occasion, a stretching force F13is exerted to a point P7of starting to contact with the drive stretching member22. When a rotational driving torque at this occasion is designated by notation T2(N·m), a diameter of moving the drive stretching member is designated by notation R3(m), and a tension force exerted to the drive stretching member22is designated by notation F19(N), F13=(T2/R3)+F19(N). Further, according to the embodiment, the tension force F13is selected to 53 N.

Next, a stretching force F14is exerted to a point P8at which the intermediate transferring belt23starts separating from the stretching member21a. Meanwhile, a stretching force F15is exerted to a point P9at which the intermediate transferring belt23starts contacting with the stretching member21a. Here, a stretching force F14is equal to the stretching force F13. The stretching force F15is equal to F23(N) which is a synthesized force of the stretching force F14and a tension force F20of the stretching member21a. Therefore, as magnitude of force, F15=F14=F13.

Also with regard to the stretching member21b, similarly, stretching forces are F17=F16=F15. As in the case of the stretching force F15discussed above, the stretching force F17is equal to F24(N) which is a synthesized force of the stretching force F16and a tension force F21of the stretching member21b. Further, a reaction force F18(N) is exerted to point P12at which the intermediate transferring belt23starts separating from the stretching member21c,and a tension force exerted to the stretching member21cis designated by notation F22(N). As magnitude of force, reaction force F18=stretching force F17. Further, between the point P12and a point P13, similar to the case at the point P3and the point P5ofFIG. 1, the stretching force and the friction force received by the intermediate transferring belt23from the stretching member21care canceled by each other and a force of stretching the intermediate transferring belt23is further reduced. The same goes with an interval between a point P14and a point P7. Therefore, also the force of stretching the intermediate transferring belt23is further reduced also between the point P13and the point P14.

Summarizing the above-described, at intervals among the points P7, P8, P9, P10, P11and point P12ofFIG. 11, is F13=(T2/R3)+F19(N). Meanwhile, at intervals among the points P12, P13, P14, and point P7, the force of pulling the intermediate transferring belt23is smaller than the stretching force F13. The stretching force is further reduced on the downstream side of the drive stretching member22in the belt rotating direction at an interval to a contiguous one of the stretching members.

An explanation will be given of an example of incorporating the above-described intermediate transferring unit20to an image forming apparatus30. InFIG. 12, numeral31designates a developing unit which is provided with a developing rotary. The developing rotary is rotated in an arrow E direction. Inside of the developing rotary is divided in four and respective divisions are provided with image forming units of 4 colors of yellow (Y), cyan (C), magenta (M) and black (Bk). In the example of yellow (Y), a developing roller31aand a toner supply roller31b, a toner control blade31cand a toner agitator31dare provided. A similar constitution is provided for other color.

Numeral32designates light ray irradiated from an exposure unit, numeral33designates a charging unit, numeral34designates light ray irradiated from a discharging unit and numeral35designates a photosensitive member unit. The photosensitive member unit35is rotated in an arrow G direction. A primary transferring unit37ais formed by the photosensitive member unit35and the stretching member21bof the intermediate transferring unit20. Numeral36designates a cleaner unit and notation37bdesignates a secondary transferring unit which is constituted by the drive stretching member22and a transferring roller37c. A fixing unit38is provided with a heating roller38ahaving a heater H at inside thereof and a pressing roller38b.

Numeral39designates recording paper which is carried in an arrow I direction. Notation30adesignates an intermediate cleaner unit which is separated from and contacted to the intermediate transferring belt23in arrows A′ directions. The intermediate transferring belt23is circulated in an arrow D direction. Further, the transferring roller37cis separated from and contacted with the drive stretching member22in arrows B′ directions.

Next, an explanation will be given of a procedure of forming an image by the image forming apparatus30shown inFIG. 12.

(1) Assum that the intermediate cleaner unit30ais separated and the second transfer unit37bis brought into a separated state.

(2) A portion (M) for magenta color of the rotary developing unit31is opposed to the photosensitive member unit35.

(3) The photosensitive member unit35, the intermediate transferring belt23and the like start driving to rotate.

(4) The photosensitive member unit35is charged to −600V by the charging unit33.

(5) An electrostatic latent image is formed on the photosensitive member unit35by the light ray32from the exposure unit

(6) The electrostatic latent image is developed by the portion for magenta color of the rotary developing unit31.

(7) The primary transferring unit37ais applied with +700V to transfer a magenta developed image on the photosensitive member unit35onto the intermediate transferring belt23.

(8) The transfer remaining toner of the photosensitive member unit35passing the primary transferring unit37ais scraped by the cleaner unit36.

(9) Further, the light ray34from the discharging unit is irradiated and remaining electrostatic latent image on the synthesized unit35is nullified.

(10) The photosensitive member unit35is charged to −600V by the charging unit33.

(11) An electrostatic latent image is formed on the photosensitive member unit35by the exposure unit32.

(12) The rotary developing unit31is rotated and a portion (C) thereof for cyan color is opposed to the photosensitive member unit35.

(13) The electrostatic latent image is developed at the portion of the rotary developing unit31for cyan color.

(14) The primary transferring unit37ais applied with +700V to transfer a cyan developed image on the photosensitive member unit35to overlap on the intermediate transferring belt23formed with the magenta image.

(15) The transfer remaining toner of the photosensitive unit35passing the primary transferring unit37ais scraped by the cleaner unit36.

(16) Further, the light ray34from the discharging unit is made incident to nullify the remaining electrostatic latent image on the photosensitive member unit35.

(17) The photosensitive member unit35is charged to −600V by the charging unit33.

(18) An electrostatic latent image is formed on the photosensitive member unit35by the light ray32from the exposure unit.

(19) The rotary developing unit31is rotated and a portion thereof for yellow color is opposed to the photosensitive member unit35.

(20) The electrostatic latent image is developed on the photosensitive member unit35at the portion for yellow color of the rotary developing unit31.

(21) The primary transferring unit is applied with +700V to transfer a yellow developed image on the photosensitive member unit35to overlap on the intermediate transferring belt23formed with magenta and cyan images.

(22) The transfer remaining toner of the photosensitive member unit35passing the primary transferring unit37ais scraped by the cleaner unit36.

(23) The light ray34from the discharging unit is made incident and the remaining electrostatic latent image on the photosensitive member unit35is nullified.

(24) The photosensitive member unit35is charged to −600V by the charging unit33.

(25) An electrostatic latent image is formed on the photosensitive member unit35by the light ray32from the exposure unit.

(26) The rotary developing unit31is rotated and a portion (Bk) thereof for black color is opposed to the photosensitive member.

(27) The electrostatic latent image on the photosensitive member unit35is developed by the portion for black color of the rotary developing unit31.

(28) The primary transferring unit is applied with +700V to transfer a black color developed image on the photosensitive member to overlap the intermediate transferring belt23formed with magenta, cyan and yellow images and a full color image is formed on the intermediate transferring belt23.

(29) The transfer remaining toner of the photosensitive member unit35passing the primary transferring unit37ais scraped by the cleaner unit36.

(30) The light ray34from the discharging unit is made incident and the remaining electrostatic latent image is nullified.

(31) The recording paper39is carried in the arrow I direction ofFIG. 12and advances between the intermediate transferring belt23and the transferring roller37cof the secondary transferring unit37b.

(32) The transferring roller37cof the secondary transferring unit37bis brought into contact with the intermediate transferring belt23.

(33) The secondary transferring unit37bis applied with voltage for making current of +20 μA flow to transfer the full color image on the intermediate transferring belt23onto the recording paper39.

(34) The recording paper39transferred with the full color toner image is carried to the fixing unit38. At the fixing unit38, the toner image on the recording paper39is melted to fix by heat and pressure.

(35) The intermediate transferring cleaner unit30ais brought into contact with the intermediate transferring belt23.

(36) Thereby, the transfer remaining toner or paper powder on the intermediate transferring belt23passing the secondary transferring unit37bis scraped.

(37) The operation returns to (1) again in the case of continuous printing.

Other conditions are as follows. Drive torque of the drive stretching member is set to 0.25 (N·m). Therefore, the stretching force F13ofFIG. 11becomes (0.25/0.015)+53=70 N. Further, a distance of points P7, P8, P9, P10, P11, P12ofFIG. 11is 347 mm. Rotational speed of the intermediate transferring belt23is 215 mm/sec in surface speed and paper passing speed is 10 ppm of A4 paper passing transversely.

Next, an explanation will be given of a relationship between a length and durability of the seam portion. The durability is evaluated by changing the length of the seam portion and carrying out continuous printing by the image forming apparatus shown inFIG. 12. A printed image is a full color character image of A4 size. In evaluating the durability, the image forming apparatus is stopped at each continuous printing of 500 sheets and a lid of the apparatus is opened to observe with eyes whether the damage of exfoliation, float-up, crack, break or the like is present at the seam portion of the intermediate transferring belt. Further, a total number of sheets of passing paper at a time point of bringing about the damage is defined as a life printing sheet number.

Table 2 shows a result of evaluating the length of the seam portion and the life printing sheet number.

TABLE 2length of seam portion (mm)life printing sheet number36750500357495003475000033729500327290003172750050belt is cut immediately30belt is cut immediately

As shown in Table 2, although there is more or less measurement dispersion, by making the seam length longer than he distance 347 mm of points P7, P8, P9, P10, P11, P12ofFIG. 11, shear force exerted to the seam portion can be reduced. Further, the life printing sheet number can be increased.

When at least one of the primary transferring unit, the secondary transferring unit and the intermediate cleaning unit in the image forming apparatus ofFIG. 12is a contact-type device (including a device which is brought into contact therewith when the transfer or the cleaning is performed and separated therefrom in other case), a force of driving to circulate the intermediate transferring belt needs to be higher than that in the non-contact type. Therefore, also the shear force applied to the seam portion is increased and a degree of breaking the intermediate transferring23is also increased. However, since the shear force can be reduced by the constitution of the invention, the effect of preventing the intermediate transferring belt23from breaking can further be enhanced.

In this embodiment, the image forming apparatus shown inFIG. 12is constructed by a constitution in which the intermediate transferring belt23as explained in reference toFIG. 10runs on a transporting path in the trapezoidal shape. However, the intermediate transferring belt can also be configured by a constitution of being stretched between the drive stretching member and the stretching member as shown inFIG. 1. In this case, for example, the drive stretching member is arranged to be opposed to the photosensitive member unit34.

As a third embodiment, an explanation will be given of an example of applying a seamed endless belt according to the invention as a fixing belt in an image forming apparatus.

i) As a substrate, a polyimide film having a thickness of 200 μm, a width of 340 mm and a length of 122 mm is used.

iii) The fixing belt may be made conductive to escape static electricity with an object of preventing the toner from being scattered by electrostatic repulsion in fixing. In this case, as a conductive additive agent, perchlorates, or a compound made conductive by doping antimony, indium or the like to each of zinc oxide, tin oxide, antimony oxide, titanium oxide, or metal particles or metal fibers of Cu, Al, Ni, stainless steel, iron or carbon fiber or the like can be used.

iv) An overlapped portion for partially overlapping both longitudinal ends of the film is formed (seeFIG. 2) and the overlapped portion is adhered by an adhering agent (KS9100; Hitachi Chemical Co., Ltd.). Otherwise, the film may be welded to adhere by heat and pressure.

v) A length of the seam portion is 22 mm and a diameter of the formed fixing belt is φ32 mm.

FIG. 13shows an example of constituting a fixing unit40by using the seamed endless belt according to this embodiment. In this figure, numeral41designates a fixing belt and numeral42designates a heating member provided with a heat generator H and serving as a drive stretching member. Numeral44designates a pressing member and numeral43designates a stretching member. Also, a recording medium45is supplied in a direction K to a position between the belt41and the pressing member44.

As the drive stretching member42, a surface of a pipe made of aluminum having an outer diameter of φ18 mm, a wall thickness of 1 mm and a length of 372 mm is coated with silicone rubber having a thickness of 300 μm. Further, a halogen lamp of 1050 W is arranged as the heat generator at inside thereof. The stretching member43is a pipe made of aluminum having an outer diameter of φ18 mm, a wall thickness of 1 mm and a length of 372 mm. As the pressing member44, a surface of a pipe made of aluminum having an outer diameter φ18 mm, a wall thickness of 1 mm and a length of 372 mm is coated with a PFA tube having a thickness of 30 μm. A distance between a center of the drive stretching member42and a center of the stretching member43is set to 22 mm. Further, the drive stretching member42and the pressing member44are pressed by a total load of 3 kg.

As has been explained of operation of force in reference toFIG. 1, the fixing belt41is exerted with a stretching force and a reaction force thereof between points P15and P16. Meanwhile, among points of P16, P17, P18, P15, a force of stretching the fixing belt is further reduced. Further, a tension force between the drive stretching member42and the stretching member43is 15 N and drive torque of the drive stretching member42is 0.1 N·m. Therefore, the stretching force of the fixing belt exerted between the points P15and P16becomes (0.1/0.009)+15=26 N.

Next, an explanation will be given of an example of incorporating the above fixing unit explained in place of the constitution of the fixing unit38of the image forming apparatus30shown inFIG. 12. Here, a distance between the points P15and P16is 22 mm. Further, the circulation speed of the fixing belt41is set to 215 mm/sec as surface speed and paper passing speed is set to 10 ppm of A4 paper passing transversely and fixing temperature is set to 190° C.

An explanation will be given of a relationship between a length and durability of a seam portion according to the embodiment. The durability is evaluated by changing the length of the seam portion. A printed image is a full color character image of A4 size. In evaluating the durability, the image forming apparatus is stopped at each continuous printing of 500 sheets and a lid of the apparatus is opened to observe with eyes whether there is damage of exfoliation, float-up, crack, break or the like at the seam portion of the intermediate transferring belt. Further, a total seat number of passing paper at a time point of bringing about such a damage is defined as a life printing sheet number.

Table 3 shows a result of evaluating the length of the seam portion and the life printing sheet number.

TABLE 3length of seam portion (mm)life printing sheet number42300003230000223100012belt is cut immediately2belt is cut immediately

As shown in Table 3, although there is more or less measurement dispersion, by making the seam length longer than the distance 22 mm between the points P15and P16ofFIG. 13, shear force exerted to the seam portion can be reduced. Further, the life printing sheet number can be increased.

The following advantages are obtained according to the embodiment.

i) The higher the temperature of a portion of the seam portion for melting to adhere resin or adhering agent or the like, the lower the adhering strength. Therefore, in the case of the fixing belt used at a temperature higher than that of the photosensitive belt member or the intermediate transferring belt used at room temperature, a force of stretching the fixing belt needs to be further reduced. According to the embodiment, since the force of stretching the fixing belt can be reduced as described above, the adhering strength can be maintained.

ii) Further, by using epoxy resin, urea resin or a thermosetting compound added therewith as the adhering agent, the high adhering strength can be maintained even at high temperature. When the compound is used along with the invention, a fixing belt having longer life can be realized. The fixing unit explained in reference toFIG. 13can be used in place of the fixing unit16of the image forming apparatus described inFIG. 9. The endless belt1ofFIG. 9in this case having the length of the seam portion of the constitution of the invention can be used. Further, a constitution of a prior art can also be constituted thereby.

As a fourth embodiment, an explanation will be given of another example of applying a seamed endless belt according to the invention as a fixing belt in an image forming apparatus.

i) As a substrate, a polyimide film having a thickness of 200 μm, a width of 340 mm and a length of 129 mm is used.

iii) Further, a fixing belt may be made conductive to escape static electricity with an object of preventing a toner from being scattered by electrostatic repulsion in fixing. In this case, as a conductive additive agent, perchlorates, or a compound made conductive by doping antimony, indium or the like to each of zinc oxide, tin oxide, antimony oxide, or titanium oxide, or metal particles or metal fibers of Cu, Al, Ni, stainless steel, iron, or carbon fiber or the like can be used.

iv) An overlapped portion is formed by partially overlapping both longitudinal ends of the film (seeFIG. 2) and the overlapped portion is adhered by an adhering agent (KS9100; Hitachi Chemical Co., Ltd.). Otherwise, the film may be welded to adhere by heat and pressure.

v) A width of a seam portion is 10 mm and a diameter of the formed fixing belt is φ37.8 mm.

A fixing unit (fixing device)50is constituted by using the seamed endless belt according to this embodiment as shown inFIG. 14. In this figure, numeral51designates a fixing belt, numeral52designates a drive stretching member, numeral53designates a stretching member in a semiannular shape, and numeral54designates a heating member having a heat generator H.

As the drive stretching member52, a pipe made of stainless steel having an outer diameter of φ25 mm, a wall thickness of 0.4 mm and a length of 372 mm is used. A surface of the pipe is coated with silicone rubber having a thickness of 300 μm.

In this way, since a material of the stretching member53uses not a metal but a resin having excellent insulating performance, the heat of the fixing belt51can be prevented from being deprived by the stretching member53. Therefore, a time period of heating the fixing belt51from a state of room temperature to desired temperature (warm up) can be shortened.

Further, since the stretching member is constituted by the semiannular shape, material cost can be reduced in comparison with that of the case of using the stretching member in a cylindrical shape as shown byFIG. 11.

In a case where the printing operations are performed with intervals, warm up of the fixing unit is repeated. At this occasion, since a time period of exposing the fixing belt51to high temperature by warm up can be shortened, thermal fatigue or thermal deterioration of the seam portion is reduced. As a result, a fixing belt having longer life can be realized.

As the heating member54, a pipe54amade of stainless steel having an outer diameter of φ25 mm, a wall thickness of 0.4 mm and a length of 372 mm is used. A surface of the pipe is coated with silicone rubber having a wall thickness of 400 μm and a PFA tube54bhaving a thickness of 30 μm is coated thereon. Further, as the heat generator at inside of the heating member, a halogen lamp of 1050 W is arranged.

A portion of the fixing belt51is made to wrap between a point P19and a point P20of the heating member54. An angle of a circular arc between P19and P20is 38°. A distance between the point P20and a point P21is 10 mm, a tension force F28(F29) between the drive stretching member52and the stretching member53is 13N and drive torque of the drive stretching member52is 0.13 (N·m). Further, the drive stretching member52and the pressing member54are pressed by a total load of 10 kg.

Next, an explanation will be given of equilibrium of force of the fixing unit shown inFIG. 14. A force F25for driving to rotate the drive stretching member52is transmitted to the heating member54via the fixing belt51.

Further, at the point P20at which the fixing belt51starts separating from the heating member54, a stretching force F26is exerted to the fixing belt51. Further, a reaction force F27is exerted to the point P21at which the fixing belt51starts contacting the stretching member53.

In this case, since the heating member54also serves as the drive stretching member, between the point P19and the point P20, the stretching force and the friction force are canceled by each other, so that a force of stretching the fixing belt51is further reduced. Therefore, the driving force is F25=(0.13/0.0125)+13=23.4 N>stretching force F26=reaction force F27. Further, the force of stretching the fixing belt51is further reduced also between points P21and P22and between points P23and P19at which the stretching force and the friction force are cancelled by each other.

In the example ofFIG. 14, there is a constructed a constitution in which: i) the heating member54is arranged at a position opposed to the drive stretching member52via the fixing belt51; ii) a portion of the fixing belt51is made to wrap the heating member54; and iii) the drive force is transmitted to the heating member54by driving the drive stretching member52.

Therefore, at a portion of the heating member54at which the fixing belt51is made to wrap, when the friction force received by the fixing belt51of the heating member54is designated by notation Fa and a resultant force of the tension force of the drive stretching member52and the stretching force received by the fixing belt51by the torque of driving to rotate the drive stretching member52is designated by notation Fb, Fa and Fb are canceled by each other. Therefore, when a stretching force Fc at the point P19at which the fixing belt51starts separating from the heating member54, Fc=Fb−Fa<Fb.

Therefore, the stretching force Fb by the drive stretching member52can be reduced by the friction force Fa of the heating member54and the maximum stretching force Fc exerted to the fixing belt51can be reduced. Therefore, shear force exerted to an adhering layer of the seam portion can further be reduced. In this way, by using the fixing belt at the fixing unit having the constitution shown byFIG. 14, a seemed belt having longer life can be realized. According to the example ofFIG. 14in the fixing apparatus having the constitution in which the heating member is arranged by being partially brought into contact with the drive stretching member and the endless belt is run along the contact portion, damage of exfoliation or break of the seam portion of the endless belt can be prevented.

Next, an explanation will be given of an example of constituting an image forming apparatus by incorporating the fixing unit50shown inFIG. 14in place of the fixing unit38shown inFIG. 12. Here, a distance between the points P20and P21ofFIG. 14is set to 10 mm. Further, circulating speed of the fixing belt51is set to 215 mm/sec by surface speed and paper passing speed is set to 10 ppm in A4 paper passed transversely and the fixing temperature is set to 190° C.

Further, also in the image forming apparatus shown inFIG. 9, the fixing unit ofFIG. 14can be used in place of the fixing unit16.

In the constitution, durability is evaluated by changing the length of the seam portion of the fixing belt51. A printed image is a full color character image of A4 size. In evaluating the durability, the image forming apparatus is stopped at each continuous printing of 500 sheets, a lid of the apparatus is opened to observe with eyes whether there is damage of exfoliation, float-up, crack, break or the like at the seem portion of the intermediate transferring belt. Further, a total seat number of passing paper at a time point of bringing about such a damage is defined as a life printing heat number.

Table 4 shows a result of evaluating the length of the seam portion and the life printing sheet number.

From Table 4, although there is more or less measurement dispersion, by making the seam length longer than the distance of 10 mm between the points P20and P21ofFIG. 14, the shear force exerted to the seam portion can be reduced. Further, the life printing sheet number can be increased.

TABLE 4length of seam portion (mm)life printing sheet number2040500154000010405008155005belt is cut immediately

As a fifth embodiment, an explanation will be given of still another example of applying a seamed endless belt according to the invention as a fixing belt in an image forming apparatus.

InFIG. 15, numeral50adesignates a fixing unit, numeral51designates a fixing belt, numeral52designates a drive stretching member, numeral53designates a stretching member in a semiannular shape, numeral54designates a heating member having a heat generator H, numeral55designates recording paper (recording medium) and numeral56designates a cleaning member.

The drive stretching member52and the stretching member53are respectively brought into contact with the heating member54. A portion of the fixing belt51is made to wrap between a point P31and a point P32of the heating member54. The heating member54serves as a pressing member for pressing the fixing belt51to the drive stretching member52. The drive stretching member52is rotated in an arrow V direction and drive force is transmitted to the heating member54to thereby rotate the heating member54in an arrow U direction.

Next, an explanation will be given of equilibrium of forces in the fixing unit shown inFIG. 15. A rotational drive force F31of the drive stretching member52is transmitted to the heating member54at the point P31. The drive force F31becomes a resultant force of a tension force F34and rotational drive torque of the drive stretching member52. Between the points P31to P32, since the drive force F31and a friction force (caused by a friction torque of the pressing member) are canceled by each other, a force of stretching the belt (stretching force F32) becomes smaller than the drive force F31.

Therefore, when a stretching force applied to the belt at the point P32is designated by notation F32, F32<F31.

Further, between the points P32and P33, the stretching force F32is further reduced by a dynamic friction force of the belt and the belt stretching member53. Therefore, when a stretching force exerted to the belt at the point P33is designated by notation F33, F33<F32.FIG. 16is a schematic view showing the stretching forces exerted to the fixing belt51at the respective points P31, P32and P33. In this figure, white circles and black circles represent that the force F32(not F33) is exerting to the point P31inFIG. 15, for example.

InFIG. 15, numeral50adesignates a fixing unit, numeral51designates a fixing belt, numeral52designates a drive stretching member, numeral53designates a stretching member in a semiannular shape, numeral54designates a heating member having a heat generator H, numeral55designats recording paper (recording medium) and numeral66designates a cleaning member.

The drive stretching member52and the stretching member53are respectively brought into contact with the heating member54. A portion of the fixing belt51is made to wrap between a point P31and a point P32of the heating member54. The heating member54serves as a pressing member for pressing the fixing belt51to the drive stretching member52. The drive stretching member52is rotated in an arrow V direction and drive force is transmitted to the heating member54to thereby rotate the heating member54in an arrow U direction.

FIG. 2is a perspective view showing an example of an endless belt used as the fixing belt51shown inFIG. 15. InFIG. 2, numeral4designates a film, numeral5designates a seam portion overlapping both end portions of the film4and the endless belt is formed by the film4. A film on an upper side of the seam portion5is designated by notation4aand a film on a lower side thereof is designated by notation4b.

FIG. 4is a schematic view showing forces exerted to the seam portion5of the endless belt. InFIG. 3, numeral6designates an adhering layer of the seam portion5. As shown byFIG. 3, the adhering layer6of the seam portion5of the endless belt is exerted with a shear force γ1by a stretching force F1and a shear force γ2by a reaction force F2.

Here, when a length of the seam portion5is designated by notation L(m) and a width of the belt (width of the adhering layer) is designated by notation W(m), γ1=F1/(L·W)=F2/(L·W)=γ2(N/m)=(Pa) is established. Therefore, it is shown that the shear force γ1(γ2) is reduced in inverse proportion to the length L of the seam portion. Further, the stretching force F1and the reaction force F2are equal to the stretching force F32shown inFIGS. 15 and 16.

Further, when a distance between the point P31and the point P32ofFIG. 15is designated by notation Lh(m), in the case of L=Lh, the stretching force F1and the reaction force F2become equal to the stretching force F33shown inFIG. 16. In view ofFIG. 16, F33<F32is established and therefore, the shear force γ1(γ2) is also reduced. Thereafter, in the case of L>Lh, the shear force is saturated.FIG. 4is an explanatory view showing a relationship between the length L of the seam portion and the shear force γ1(γ2). In view ofFIG. 4, the shear force γ1(γ2) exerted to the adhering layer6can be minimized by making the length L of the seam portion equal to or larger than the distance Lh between the point P31and the point P32.

In this embodiment, the length of the seam portion is made to be equal to or larger than the distance Lh between the point P31at which the heating member (pressing member)54is brought into contact with the drive stretching member52and the point P32at which the heating member54is brought into contact with the stretching member53.

FIG. 17is a schematic view showing forces exerted to the endless belt when a portion of the seam portion of the fixing belt is brought into contact with the stretching member53. In this figure, notation51xdesignates the adhering layer of the fixing belt, notation51adesignates one surface of the fixing belt and notation51bdesignates other surface of the fixing belt. The seam portion of the fixing belt51is disposed at a position between P31and P32ofFIG. 15and is brought into contact with the stretching member53.

At portion A at which the seam portion is not brought into contact with the stretching member53, a shear force γ33by a stretching force F38and a shear force γ35by a reaction force F40are exerted to the adhering layer51x. Further, at portion B at which the seam portion is brought into contact with the stretching member53, a shear force γ34by a stretching force F39and a shear force γ36by a reaction force F41are exerted to the adhering layer. Further, a relationship of F39<F38and F41<F40is established by a friction force received from the stretching member53.

Therefore, relationships of the forces and forces inFIG. 3are as follows.
F38+F39<F1, γ33+γ34<γ1, F40+F41<F2, γ35+γ36<γ2
That is, when a portion of the seam portion is brought into contact with the stretching member53, a shear force exerted to a total of the adhering layer51xbecomes smaller than γ1, γ2ofFIG. 3. Even when a portion of the seam portion is brought into contact with the drive stretching member52, a shear force exerted to the total of the adhering layer51xsimilarly becomes smaller than γ1, γ2ofFIG. 3.

Therefore, the shear force γ1(γ2) exerted to the adhering layer of the fixing belt can be minimized by making the length L of the seam portion equal to or larger than the distance Lh between the point P31and the point P32. The shear force γ1(γ2) exerted to the adhering layer of the fixing belt can be minimized whenever at least a part of the seam portion is disposed between the point P31and the point P32. Therefore, damage of the fixing belt can be prevented and the service life can be prolonged.

In this embodiment, it is configured that: i) the heating member54is arranged at a position opposed to the drive stretching member52via the fixing belt51; ii) a portion of the fixing belt51is made to wrap the heating member54; and iii) the drive stretching member52is driven to transmit the drive force to the heating member54.

Therefore, when at a portion of the heating member54for wrapping the fixing belt51, a friction force Fa received by the fixing belt51from the heating member54and a resultant force Fb of the tension force of the drive stretching member52and a stretching force received by the fixing belt51by a torque of driving to rotate the drive stretching member52are canceled by each other. Therefore, when a stretching force at the point P31at which the fixing belt51starts separating from the heating member54is designated by notation Fc, Fc=Fb−Fa<Fb.

Therefore, the stretching force Fb by the drive stretching member52can be reduced by the friction force Fa of the heating member54and the maximum stretching force Fc exerted to the fixing belt51can further be reduced. Therefore, the shear force exerted to the adhering layer can further be reduced. In this way, by using the fixing belt at the fixing unit having the constitution as shown byFIG. 15, a seamed belt having long service life can be realized. That is, in the fixing unit having the constitution in which the heating member is arranged by partially brought into contact with the drive stretching member and the endless belt is run along the contact portion, damage of exfoliation, break or the like of the seam portion of the endless belt can be prevented.

The endless belt in this embodiment is almost similar to that explained as the fourth embodiment. The detailed explanations are omitted and only the different matters will be described below.

A polyimide film having a thickness of 200 μm, a width of 340 mm and a length of 107 mm is used as a base material.

A length of a seam portion is 9 mm and a diameter of the formed fixing belt is φ31 mm.

The fixing unit50aof this embodiment is almost similar to that examined as the fourth embodiment. The detailed explanations are omitted and only the different matters will be described below.

A portion of the fixing belt51is made to wrap the heating member (pressing member)54(between point P31and point P32) and a length thereof is 8.6 mm. A tension force F34(F35) of the drive stretching member52and the stretching member53is 13 N, drive torque of the drive stretching member52is 0.13 N·m. Further, the drive stretching member52and the heating member54are pressed by a total load of 10 kg and the stretching member53and the heating member54are pressed by a total load of 3 kg.

FIG. 18shows an example of an image forming apparatus30aincorporating the fixing unit50aof this embodiment. Since the elements other than the fixing unit50aare identical with those inFIG. 12, the detailed explanations are omitted.

The drive torque of the drive stretching member52is set to 0.13 (N·m). Therefore, the drive force F31ofFIG. 15becomes as follows.
(0.13/0.0125)+13=23.4 N

Next, an explanation will be given of a relationship between a length and durability of the seam portion of the fixing belt. The durability is evaluated by changing the length of the seam portion and carrying out continuous printing by the image forming apparatus shown inFIG. 18. A printed image is a full color character image of A4 size. In evaluating the durability, the image forming apparatus is stopped at each continuous printing of 500 sheets and a lid of the apparatus is opened to observe with eyes whether the damage of exfoliation, float-up, crack, break or the like is present at the seam portion of the intermediate transfer belt. Further, a total number of sheets of passing paper at a time point of bringing about the damage is defined as a life printing sheet number. Table 5 shows the length of the seam portion and a result of evaluating the life printing sheet number.

TABLE 5length of seam portion (mm)life printing sheet number125950010600009595008305006225004belt is cut immediately

As shown in Table 5, although there is more or less measurement dispersion, by making the seam width longer than the length 8.6 mm between the points P31and P32ofFIG. 15, the shear force exerted to the seam portion can be reduced. Further, a life printing sheet number can be increased.

The higher the temperature of the adhering portion of the seam portion of the fixing belt for melting resin or the adhering agent or the like, the lower the adhering strength. Therefore, the fixing belt used at high temperature needs to make the force of stretching the fixing belt lower than that of the photosensitive belt member or the intermediate transfer belt used at room temperature. According to the invention, since the force of stretching the fixing belt can be reduced as described above, when the fixing device of the invention is used, the effect of preventing damage of the fixing belt can further be enhanced.

Further, by using epoxy resin, urea resin or a thermosetting compound added therewith as the adhering agent, the high adhering strength can be maintained even at high temperature. By using such an adhering agent in the fixing device of the constitution of the invention, a fixing belt having longer service life can be realized.

FIG. 19shows another example of an image forming apparatus10aincorporating the fixing unit50aof the fifth embodiment. Since the elements other than the fixing unit50aare identical with those inFIG. 9, the detailed explanations are omitted.

Next, a sixth embodiment of the invention will be described with reference toFIG. 20. In this figure, an endless belt1is formed by winding a substrate4. A seam portion5indicated by a hatched portion is formed by adhering a portion of the wound substrate4overlapping an inner side layer4aand an outer side layer4cthereof. A length of the seam portion is made to be equal to or larger than a length Lx of a circumference of a wound portion4bof the substrate4, that is, equal to or larger than an amount of the circumference.

In this way, according to the belt for an image forming apparatus of the invention, the seam portion is formed not only at both longitudinal end portions of the substrate as in the above embodiments but the seam portion is formed by the length equal to or larger than the amount of the circumference of the wound substrate4. Therefore, even when the endless belt is stretched between stretching members to be circulated, a sufficient strength is achieved and the endless belt can be prevented from being destructed.

FIG. 21is an enlarged view of the seam portion of the endless belt1. In this figure, notations6aand6bdesignate an adhering layer. When a thickness of the substrate is designated by notation ds and a thickness of the adhering layer is designated by db, a thickness tb of the belt and a stepped difference ls are expressed as follows.
tb=2ds+db(1)
ls=ds+db(2)

FIG. 22shows a comparative example in which a seam portion is shorter than an amount of a circumference of the endless belt. In this figure, an endless belt1ais formed with a seam portion5ashorter than a circumference thereof. A hatched portion designates an adhering layer of the seam portion5a.

FIG. 23is an enlarged view of the seam portion5aofFIG. 22. In this figure, notations4aand4bdesignate a substrate at an overlapped portion and notation6adesignates an adhering layer. When a thickness of the substrate4ais designated by notation ds and a thickness of the adhering layer6ais designated by notation db, a thickness of an endless belt is provided with two levels. When the belt thickness of the seam portion at a thin level is designated by notation tb1, the belt thickness of the seam portion at a thick level is designated by notation tb2and a stepped difference is designated by notation ls, the following equations are established.
tb1=ds(3)
tb2=2ds+db(4)
ls=ds+db(5)

Here, when the endless belt is used by being stretched between stretching members to drive to circulate, the belt member needs to be provided with a predetermined thickness or more in order to achieve a desired strength. When the belt member is provided with the desired strength or less, a time period of use (service life) until the belt member is cracked or broken is shortened.

FIG. 24is an explanatory view showing an example of using the endless belt. The endless belt1is stretched between stretching members2and3. Further, notations106athrough106ddesignate butting members provided at end portions of the respective stretching members2and3in order to prevent the endless belt1from shifting to one end. In the case of the example ofFIG. 24, when the strength of the endless belt1is equal to or less than the desired strength, there is brought about a drawback that an end portion of the endless belt1is butted to the butting members106athrough106dto buckle, wrinkle or the like.

Now, when a predetermined thickness tb of the endless belt is made to be equal to or larger than 300 μm and the thickness db of the adhering layer is made to be 1 μm, the thickness ds of the substrate becomes as follows from Equation (1),
ds>149.5 μm
and the thickness of the substrate needs to be equal to or larger than 149.5 μm.

Meanwhile, from Equations (3) and (4), in the case of the belt having the constitution ofFIG. 23, a relationship between the thickness tb1of the endless belt and the thickness tb2of the endless belt at the seam portion becomes as follows,
tb2>tb1=ds>300 μm

and the thickness of the substrate needs to be equal to or larger than 300 μm.

In this embodiment, from Equation (2), each stepped difference becomes as follows.
ls=ds+db=149.5 μm+1 μm=150.5 μm
In contrast thereto, according to the constitution ofFIG. 23, from Equation (5), each stepped difference becomes as follows.
ls=ds+db=300 μm+1 μm=301 μm
Therefore, according to the constitution of the invention, in comparison with the example inFIG. 23, the stepped difference can be reduced with regard to the predetermined belt thickness in order to achieve a necessary strength.

When the necessary strength of the belt is designated by notation x(μm) in order to expand the above-described explanation to general theory, the following equations are established as follows. From Equations (1) and (2),
ds>(x−db)/2  (6)
ls>(x−db)/2+db(x+db)/2  (7)
Meanwhile, from Equations (3) and (4),
tb2>tb1=ds>x(8)
From Equation (5),
ls>x+db(9)
Therefore, when the necessary thickness of the sheet substrate is made to be equal to or larger than x(μm), each stepped difference becomes (x+db)/2 (μm) in this embodiment, and becomes equal to or larger than x+db(μm) in the case of the comparative example. Therefore, according to this embodiment, the thickness of the stepped difference can be halved.

FIG. 25shows an endless belt according to a seventh embodiment of the invention in which the stepped difference is further reduced. In this embodiment an endless belt1is formed by wounding the substrate4with a plurality of turns. Therefore, a plurality of layers of the seam portion5are formed between the substrates4.

Generally, when an endless belt wound with a circumference thereof by n times is constituted (n=2 in the example ofFIG. 20), the thickness tb and the thickness is of the stepped difference of the endless belt become as follows.
tb=nds+(n−1)db(10)
ls=ds+db(11)
When the necessary thickness of the substrate is made to be equal to or larger than x(μm), the following is established from Equation (10).
ds>[x−(n−1)db]/n(12)
Further, from Equation (11), the following is established.
ls>[x−(n−1)db]/n+db=(x+db)/n(13)
By comparing with Equation (9), the stepped difference of the endless belt of this embodiment can be made to be 1/n of that of the endless belt having the constitution ofFIG. 23.

FIG. 26shows an endless belt according to an eighth embodiment of the invention in which the stepped difference of the endless belt ofFIG. 20is further reduced. In this figure, notations4tand4rdesignate both longitudinal end portions of the substrate4, notations4sand4udesignate wound portions of the substrate4and notation4vdesignates a stepped portion. Also, in this figure, notations6rand6sdesignate an adhering layer.

In this embodiment, the stepped difference is reduced by arranging the both longitudinal end portions4tand4rto be opposed to each other through the stepped portion4v, and flattening the stepped portion4vby thermal pressing in the thickness direction thereof. Therefore, the stepped difference is reduced without entirely pressing the seam portion. When the belt is used as a fixing film, since temperature transfer to a recording medium is not reduced, a failure in fixing can be prevented from being brought about. Further, the stepped difference can be reduced to be ⅓ of the stepped difference of the case where the substrate is wound by three times as inFIG. 25.

A photosensitive film of the sixth embodiment can be obtained by the same way as described in connection with the first embodiment. Only the different matters will be described below.

As a substrate, a film of polyester resin having a predetermined thickness, a width of 340 mm and a length of 377 mm.

The photosensitive film is coated with an adhering agent (406; Loctite Corporation) over an entire face thereof while leaving a length of 188 mm and wound to adhere as shown byFIG. 20. Further, the charge transporting layer is disposed on an outer side of the seamed endless belt. A diameter of the formed seamed endless belt photosensitive member is φ60 mm.

Further, as a comparative example, a conductive layer, an under coating layer, a charge generating layer and a charge transporting layer are similarly formed at a film of polyester resin having a predetermined thickness, a width of 340 mm and a length of 243 mm and wound to adhere as shown byFIG. 22to thereby form a photosensitive film. A width of a seam portion is 55 mm and a diameter of a belt photosensitive member is φ60 mm.

Next, an explanation will be given of a relationship among the thickness, and the strength of the substrate and the image quality. Continuous printing of ten thousands sheets is carried out by the image forming apparatus shown inFIG. 9by changing the thickness of a polyester rein film which is the substrate. With regard to the strength of the substrate, it is investigated whether a damage of crack, break, buckle by one side shifting or the like is brought about at the belt member. In evaluating, the image forming apparatus is stopped at each continuous printing of 500 sheets and eye observation is carried out by opening a lid of the apparatus. With regard to the image quality, a solid image of gray is printed by A4 size and eye observation is carried out with respect to several sheets of printing at an initial stage on whether a nonuniformity of image caused by the stepped difference of the seam portion (black streak, white depletion, gross streak) or the like is brought about.

Table 6 shows experimental results of the above evaluation. It is found that although there is a region excellent in the strength and the image quality in the embodiment, while the region is not present in the comparative example.

The endless belt according to the seventh embodiment can be obtained as follows. A photosensitive film is formed by forming a conductive layer, an under coating layer, a charge generating layer and a charge transporting layer above a film of polyester resin having a predetermined thickness, a width of 340 mm and a length of 565 mm similar to the above-described. The photosensitive film is coated with an adhering agent (406; Loctite Corporation) over an entire face thereof while leaving a length of 188 mm and wound as shown byFIG. 25to adhere. Further, the charge transporting layer is disposed on an outer side of the seamed endless belt.

Ten thousands sheets of continuous printing is carried out by the image forming apparatus shown inFIG. 9by changing the thickness of the polyester resin film constituting the substrate. With regard to the strength of the substrate, it is investigated whether the damage of crack, break, buckling due to one side shifting or the like is brought about at the belt member. In evaluating, the image forming apparatus is stopped at each continuous printing of 500 sheets and eye observation is carried out by opening the lid of the apparatus.

With regard to the image quality, a solid image of gray of A4 size is printed and eye observation is carried out with respect to several sheets for printing at an initial stage on whether image nonuniformity caused by the stepped difference of the seam portion (black streak, white depletion, gross streak) or the like is brought about.

Table 7 shows experimental results of the above evaluation. When embodiments of Table 7 and Table 6 are compared, it is found that the region excellent in the strength and the image quality can be widened in the case of the substrate wound by a number of turns.

The endless belt according to the eighth embodiment can be obtained as follows. A photosensitive film is formed by forming a conductive layer, an under coating layer, a charge generating layer and a charge transporting layer above a film of polyester resin having a predetermined thickness, a width of 340 mm and a length of 377 mm similar to the above-described. The photosensitive film is coated with an adhering agent (406; Loctite Corporation) over an entire face thereof while leaving a length of 188 mm and wound as shown byFIG. 20and adhered by separating the longitudinal end portions thereof by about 100 μm. In this case, the charging transporting layer is arranged to be disposed on the outer side of the belt.

Next the separated portion is mounted on a hot plate, placed with a flat plate from above, applied with a total load of 60 kg and heated for 30 minutes at 180° C. When the selection after processing was observed by a microscope, the section was as shown byFIG. 26.

Ten thousand sheets of continuous printing is carried out by the image forming apparatus shown inFIG. 9by changing the thickness of the polyester rein film constituting the substrate. With regard to the strength of the substrate, it is invested whether a damage of crack, break, buckle by one side shifting or the like is brought about at the belt member. In evaluating, the image forming apparatus is stopped at each 500 sheets of continuous printing and eye observation is carried out by opening the lid of the apparatus.

With regard to the image quality, eye observation is carried out with respect to several sheets of printing at an initial stage on whether image nonuniformity caused by the stepped difference of the seam portion (black streak, white depletion, gross steak) or the like is brought about. Table 8 shows experimental results the evaluation. When Table 8 is compared with Table 6, it is found that the region excellent in the strength and the image quality in the eighth embodiment is widened in comparison with the sixth embodiment.

As a ninth embodiment, an explanation will be given of an example in which an endless belt of the invention is used as an intermediate transfer belt in an image forming apparatus. The endless belt can be obtained by almost the same way as described in connection with the second embodiment. Only the different matters will be described below.

As a substrate, there is used a conducive resin film having a thickness of 80 μm, a width of 340 mm and a length of 2512 mm.

The film is coated with an adhering agent (1521; Three Bond Co., Ltd) while leaving a length of 628 mm, wound by a plurality of turns as shown byFIG. 25or wound by an amount of a total of four circumferences to adhere. Otherwise, the film may be melted to adhere by heat and pressure. A diameter of the formed seamed endless intermediate transfer belt is φ200 mm.

A comparative example is formed as follows: i) As a substrate, a conductive resin film (dispersed with 20 wt % of carbon black powder as a conductive agent in polyurethane resin) having a thickness of 300 μm, a width of 340 mm and a length of 975 mm is used; ii) The film is formed with an overlapped portion in which both longitudinal end portions are overlapped (seeFIG. 22). The overlapped portion is mounted on a hot plate, placed with a flat plate from above, applied with a total load of 60 kg and heated for 30 minutes at 290° C.; iii) When the section after processing was observed, the film was as shown byFIG. 32; iv) A length of a seam portion is 347 mm and a diameter of the formed seamed endless intermediate transfer belt is φ200 mm.

Next, an explanation will be given of evaluating an image quality concerning a surrounding environment when the intermediate transfer belt is used. Printing carried out by the image forming apparatus shown inFIG. 12with regard to three levels of the surrounding environment of LL (10° C., 15% humidity), NN (25° C., 60% humidity) and HH (35° C., 65% humidity).

10 sheets of A3 size of an image of a solid image of gray of only magenta are continuously printed and eye observation is carried out on whether a nonuniformity in image (nonuniformity in density, black streak, white depletion, gross streak) caused by the seam portion is brought about. Further, when the belt thickness is made to be equal to or larger than 300 μm, there is not a damage of buckling or crack or the like and the belt strength was guaranteed. Further, since it had already been known that when the stepped difference was made to be equal to or smaller than 90 μm, a nonuniformity in an image by the stepped difference is not brought about, an experimental data thereof will be omitted. Table 9 shows experimental results of the evaluation.

It seems that occurrence of a nonuniformity in the image density (the no good result) in the comparative example is caused by a resistance of the intermediate transfer belt. Thus, the resistance of the intermediate transfer belt is measured under a condition of constant voltage of 250V by using a high resistance measuring apparatus (Hiresta; Mitsubishi Chemical Corporation). Table 10 shows the experimental results.

It is found that the resistance of the seam portion becomes larger than that of the other portion. When the belt of the Comparative example is formed, since the seam portion is compressed by heat and pressure, the density at the portion is increased and the resistance value is also increased. Further, in the environment of increasing the resistance value of the LL environment, it seems that the resistance of the seam portion exceeds an upper limit value and voltage drop is increased, so that the transferring efficiency is reduced and an amount of the toner to be transferred is reduced.

Conversely, since the resistance value is small at other than the seam portion, and the resistance value at other than the seam portion becomes less than a lower limit value in the environment of reducing the resistance value of the HH environment. As a result, it seems that discharge due to extra transfer bias is generated at other than the seam portion and scattering of the toner (no good result) is brought about. Therefore, it is found that the embodiment is easier to confine the resistance value of the intermediate transfer belt in an excellent region than the comparative example.

As a tenth embodiment, an explanation will be given of an example in which an endless belt of the invention is used as a fixing belt in an image forming apparatus. The endless belt can be obtained by almost the same way as described in connection with the third embodiment. Only the different matters will be described below.

As a substrate, a polyimide film having a thickness of 70 μm, a width of 340 mm and a length of 292 mm is used.

The film is coated with an adhering agent (KS9100; Hitachi Chemical Co., Ltd.) over an entire face thereof while leaving a length of 97 mm and wound by a plurality of turns as shown byFIG. 25to adhere. Otherwise, the film may be melted to adhere by heat and pressure. A diameter of the formed fixing belt is φ31 mm.

A comparative example is formed as follows: i) As a substrate, a polyimide film having a thickness of 20 μm, a width of 340 mm and a length of 107 mm is used; ii) The film is formed with an overlapped portion in which both longitudinal end portions are overlapped (seeFIG. 22). The overlapped portion is mounted on a hot plate, placed with a flat plate from above, applied with a total load of 80 kg and heated for 30 minutes at 290° C.; iii) when the section after processing was observed by a microscope, the film was as shown byFIG. 32; iv) A length of the seam portion is 9 mm and a diameter of the formed fixing belt is φ31 mm.

Printing is carried out by the image forming apparatus shown inFIG. 18and the image quality is evaluated. With regard to an image, 10 sheets of A3 size of a solid image of gray only of magenta are continuously printed and eye observation is carried out on whether a nonuniformity in image (nonuniformity in density, black streak, white depletion, gross streak) caused by the seam portion is brought about. Further, also fixing strength of a portion in correspondence with a seam portion of the image and a portion which is not in correspondence therewith are examined.

There is taken a ratio of densities before and after rubbing by a sand matrix eraser rubber (GAZA; Lion Office Products Corp.). The ratio is defined as a fixing rate and a fixing rate less than 0.75 is defined as a failure in fixing. Further, since it had already been known that there was not buckling, crack or the like and the belt strength was guaranteed in a case where the belt thickness was equal to or larger than 200 μm. Further, when the stepped difference was made to be equal or smaller than 90 μm, a nonuniformity in image by the stepped difference was not brought about, experimental data in such conditions will be omitted. Table 11 shows experimental results of the above evaluation.

A failure in fixing was brought about at a portion in correspondence with the seam portion of the belt of the comparative example. The fixing rate is 0.4 through 0.6. In order to investigate the cause, in the image forming apparatus ofFIG. 18, a surface of the belt at a vicinity of a sheet discharging port is measured by a radiation pyrometer (THI-700S; Tasco Japan Inc.). Temperature drop of 20° C. through 40° C. was measured in correspondence with the seam portion when paper passes the fixing unit.

FIG. 27is a diagram for explaining the temperature conductivity of substance. In this figure, numeral60designates a substance, notation Ta designates absorbing temperature and notation Tb designates radiation temperature. With regard to Equation (14), consider boundaries p0and p1having a length therebetween Lh, for example, a distance of 1 m at inside of the substance60. In this case, in the case in which a temperature rise rate of a hatched portion between the boundaries is Δt/Δτ (° C./sec), when a temperature gradient at the boundary p0is defined as (Δt/Δx)p0(° C./m) and a temperature gradient at the boundary p1is defined as (Δt/Δx)p1(° C./m), the following equation is established.
(Δt/Δx)p1−(Δt/Δx)p0=(1/a)(Δt/Δτ)  (15)

From Equation (15), the more increased is the temperature conductivity “a”, the smaller the difference between the temperature gradients of the boundaries p0and p1. That is, the smaller the temperature difference between the boundaries p0and p1. Therefore, the larger the temperature conductivity, the faster the temperature transfer of the substance. Further, in the case of forming a sheet by the substance, when a rear side of the sheet is heated, the heat immediately reaches a front side of the sheet and the temperature difference between the both sides of the sheet can almost be nullified.

Here, from Equation (14), the temperature conductivity a is inversely proportional to the density ρ of the substance. At the seam portion of the fixing plate of the comparative example, since the seam portion is compressed by heat and pressure, the density at the portion is increased. When the seam portion was cut out and the density was measured (calculated by measuring dimensions and weight), the density becomes 1.9 times as large as that of a portion other than the seam portion. It seems that the temperature conductivity of the seam portion becomes about a half or more of that of the other portion.

Next, an explanation will be given of a mechanism of bringing about a failure in fixing when the temperature conductivity is small.FIG. 28shows an enlarged view of a nip portion of the fixing unit ofFIG. 15. Immediately before fixing recording paper, the heating member54is maintained at desired temperature by the heat generator H ofFIG. 15. At the same time, by driving to rotate the fixing belt51by the drive stretching member52when paper is not passed, the fixing belt51is heated via the nip and maintained at the desire temperature.

Also temperatures of the drive stretching member52and the stretching member53stretching the fixing belt51become higher than room temperature. Under this condition, the recording paper55is carried in an arrow J direction ofFIG. 15and advances into the nip portion. InFIG. 28, numeral57designates a toner layer. At this occasion, since temperatures of the toner layer57and the recording paper55are lower than that of the heating member54, heat is conducted from the heating member54to the toner layer57in an arrow Qa direction.

At the same time, heat is transmitted also from the fixing belt51and the drive stretching member52to the toner layer57via the recording paper55from an arrow Qb direction. At this occasion, in the case of the fixing belt of the comparative example, since the seam portion is formed with a high density portion58and the temperature conductivity of the high density portion58is low, heat Qc conducted from the drive stretching member becomes lower than the heat Qb at a portion other than the seam portion. Therefore, a heat transfer amount of the seam portion becomes deficient, so that a sufficient amount of the toner layer cannot be melted and the failure in fixing is brought about.

As an eleventh embodiment, there will be explained an example in which an endless belt of the eighth embodiment is used in a fixing unit50bshown inFIG. 29.

As the driving member52, silicone foam having a wall thickness of 6 mm is provided by a length of 360 mm on a shaft52amade of stainless steel having a length of 397 mm and a diameter of φ23 mm and a PFA tube having a wall thickness of 30 μm is covered further thereon to form an outer side layer52b.

There is used a supporting member54xmade of PTFE resin in a shape of a circular arc having an outer radius of curvature of 31 mm, a wall thickness of 4 mm and a length of 360 mm. Further, a portion thereof in contact with the driving member52is formed with a flat face54y. The supporting member54xuses an electric heater of 1050 W as the heat generator H and is arranged to be opposed to the driving member52via the fixing belt51. The heat generator H is provided at inside of the supporting member54. The driving member52and the supporting member54xare pressed by a total load of 16 kg. Notation M designates a rotational direction of the driving member52and notation N designates a rotational direction of the fixing belt51.

The fixing unit50bof this embodiment is installed in place of the fixing unit50aof the image forming apparatus shown inFIG. 18. Here, circulation speed of the fixing belt51is set to 250 mm/sec in surface speed, paper passing speed is set to 10 ppm for A4 paper passed transversely and fixing temperature is set to 190° C.

Printing was carried out in this condition and image quality was evaluated. As an image, 10 sheets of A3 size of a solid image of gray only of magenta were continuously printed and eye observation was carried out on whether a nonuniformity of image caused by the seam portion (nonuniformity of density, black streak, white depletion, gross streak) or the like was brought about.

Further, fixing strengths of a portion of the image in correspondence with the seam portion and other portion were also examined. A ratio of densities before and after rubbing by a sand matrix eraser rubber (GAZA; Lion Office Products Corp.) was sampled. The ratio was defined as a fixing rate and a rate less than 0.75 was defined as a failure in fixing. Further, it had already been known that there was not buckling, crack or the like and the belt strength was guaranteed in a case where the belt thickness was made to be equal to or larger than 200 μm. Further, a nonuniformity in image by the stepped difference was not brought about when the stepped different was made to be equal to or smaller than 90 μm. Therefore, experimental data in such conditions are omitted. Table 12 shows experimental results of the above evaluation. The comparative example was the same as shown in Table 11.

As shown, low temperature offset was brought about at a portion of the belt in correspondence with the seam portion of the comparative example. The low temperature offset is a phenomenon in which the toner is not melted at all but exfoliated from above paper and adhered to a surface layer of the fixing belt. Further, a temperature of a surface of the belt at a vicinity of a paper discharge outlet is measured by a radiation pyrometer (THI-700S; Tasco Japan Inc.) Temperature drop of 60° C. through 100° C. is measured in correspondence with the seam portion when paper passes the fixing unit.

FIG. 30is an enlarged view of a nip portion of the fixing unit ofFIG. 29. Immediately before fixing the recording paper55, the fixing belt51is maintained at desired temperature by the heat generator H. At the same time, by driving to rotate the fixing belt51by the driving member52when paper is not passed, the driving member52is heated via the nip and is maintained at the desired temperature. Under the state, the recording paper55is carried in an arrow Z direction ofFIG. 29and advances into the nip portion to bring about a state shown inFIG. 30.

In this case, since temperatures of the toner layer57and the recording paper55are lower than that of the heat generator H, heat is transferred from the heat generator H to the toner layer57via the fixing belt51in the arrow Qa direction. At the same time, heat is also transferred from the driving member52to the toner layer57via the recording paper55. In the case of the comparative example, the high density portion58is formed at the seam portion of the fixing belt51, since the temperature conductivity of the high density portion58is low, heat Qd transmitted from the heat generator becomes smaller than the heat Qa at a portion other than the seam portion.

Therefore, even when the fixing unit is constituted as shown byFIG. 29, according to the comparative example, a heat transfer amount of the seam portion becomes deficient so that the toner layer57cannot be melted to bring about the low temperature offset. In this case, the temperature drop at the seam portion is more significant in the case of conducting heat from the fixing belt51to the toner layer57as shown byFIG. 30than the temperature drop in a case where heat is transferred from the fixing belt51to the toner layer57via the recording paper55, as explained in reference toFIG. 28.

The reason is that according to the example ofFIG. 30, a rate of temperature contributing to melt the toner is larger in the case of temperature of the fixing belt51in contact with the toner layer57than in the case of temperature of the driving member52in contact with the recording paper55. With regard to melting of the toner, the contributing rate of heat transfer from the driving member52in contact with the recording paper55is about a half through a third of the contributing rate of heat transfer from the fixing belt51in contact with the toner layer57. Therefore, when the fixing belt by the endless belt of the invention is used for the fixing unit as shown byFIG. 29, the effect is considerable in view of preventing the low temperature offset.

In this case, the intermediate transfer belt23shown inFIG. 18may be constituted similarly to the fixing belt shown inFIG. 20orFIG. 26.

Although the four-cycle color image forming apparatus using the developing rotary is shown in some of the above embodiments, the invention is applicable to a tandem-type color image forming apparatus. Further, the invention is applicable also to a fixing device of an image forming apparatus having a photosensitive drum as an image carrier. In this way, the invention is widely applicable to an image forming apparatus having an image carrier for transferring an image onto a recording medium.