JP4725018B2 - Fixing device - Google Patents

Description

  The present invention relates to a fixing device that is used in an image forming apparatus such as a printer, a copying machine, and a facsimile machine, and heats and melts an unfixed toner image carried on a recording medium and fixes the image on the recording medium.

  In general, the process of fixing a toner image in an image forming apparatus using powdered toner is performed after the toner image is electrostatically directly transferred onto a recording medium or after the toner image is primarily transferred to an intermediate transfer member. A method is widely employed in which after a secondary transfer to a recording medium, the recording medium is sandwiched between a heating member and a pressure member, and the toner image is heated and pressure-bonded to the recording medium.

  Examples of the fixing device include a fixing roll having a heating element such as a halogen lamp inside a cylindrical metal core and a pressure roll pressed against the fixing roll. A recording medium carrying an unfixed toner image is sandwiched between the rolls and heated and pressurized.

  Since it is difficult for the fixing device as described above to reduce the heat capacity of the fixing roll, even if the image forming apparatus is turned on and energized to the halogen lamp that is the heat source of the fixing apparatus, It takes a considerable waiting time until the temperature rises to a predetermined fixing temperature from a cold state, and lacks quick start performance. Further, in the standby state of the image forming apparatus (during non-image output), it is necessary to energize the halogen lamp and maintain the fixing roll at a predetermined temperature so that the image forming operation can be executed at any time. Is big.

  On the other hand, instead of the fixing roll, there is a type in which an endless fixing belt is used as a fixing member. The fixing belt is of a type stretched by a plurality of support rolls, and has a pressing member inside and is not stretched. In this state, there is a type in which it is driven around by pressing against a pressure roll. Since the fixing belt has a thin heat-resistant resin as a base layer and has a smaller heat capacity than the roll-shaped member, it can warm up in a shorter time than the roll-shaped member. Furthermore, the non-stretching type fixing belt can reduce the contact area with other members, and heat transfer to the other members is reduced. For this reason, more efficient warming up can be performed.

As a fixing device that drives the fixing belt in a non-tensioned state, there is one disclosed in Patent Document 1. In this fixing device, the fixing belt is heated by an electromagnetic induction heating device, and includes a conductive layer that generates heat by an induced current along the circumferential surface. A pressing member made of an elastic material is disposed inside the endless fixing belt, and presses the fixing belt strongly against the pressure roll by being pressed against the inner peripheral surface of the fixing belt by the support member. . The recording medium carrying the unfixed toner image is brought into contact with the fixing belt, is superimposed, and is sandwiched between the pressing member and the pressure roll. The toner image on the recording medium is heated by heat conduction from the fixing belt and is pressed and fixed on the recording medium.
JP 2003-84591 A

However, in the fixing device using the above-described fixing belt in the non-tensioned state, there is a problem that the following improvement is desired.
Generally, when a recording medium carrying an unfixed toner image is sent to a pressure contact portion between a heated fixing member and a pressure member and is heated and pressed and discharged, the recording medium is hardly separated from the fixing member. There is a problem. In particular, a thin recording medium, a recording medium which is left under high humidity and has a higher water content than usual, and a recording medium in which a large amount of toner is carried on a portion of several millimeters from the leading edge in the traveling direction are heated and melted. A large adhesion force is generated between the toner and the fixing member, and the recording medium that has passed through the nip portion is difficult to peel from the fixing member. Further, when a color image is formed, since a large amount of toners of a plurality of colors are carried on the recording medium, the recording medium is more difficult to peel from the fixing member.

  To solve this problem, in a fixing device of a type in which the fixing belt is stretched by a plurality of support rolls, the fixing belt is bent with a large curvature along the peripheral surface of the support roll depending on the diameter of the support roll and its arrangement. It can be peeled off from the recording medium which is going straight ahead by its own rigidity. However, the type in which the fixing belt is driven in a state of being stretched without tension has an advantage in that other members are not brought into contact with the fixing belt as much as possible, and it is difficult to bend with a large curvature by constraining the shape during the rotation driving. .

  On the other hand, if the fixing belt in a non-tensioned state is strongly pressed against the pressure roll by a pressing member fixedly supported, the pressing member made of an elastic material is repeatedly heated and pressed and compressed when the apparatus is used over a long period of time. Permanent distortion occurs and the shape changes over time. As a result, the pressing force of the fixing belt to the pressure roll and the deformation shape of the belt near the fixing nip fluctuate, and it becomes impossible to obtain an appropriate pressing force and deformation of the belt at the nip portion. May cause peeling failure.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to bring an unfixed toner image carried on a recording medium into contact with an endless fixing belt to heat and melt the recording medium. In the fixing device for fixing the recording medium, good fixing is performed over a long period of time, and the recording medium is surely peeled off from the fixing belt.

In order to solve the above-described problem, an invention according to claim 1 is a non- stretchable endless fixing belt supported so as to be capable of circular movement, a heating device for heating the fixing belt, and an outer periphery of the fixing belt. A pressure roll that is disposed to face the surface and includes an elastic layer in the vicinity of the peripheral surface, and a pressing member that is in contact with the inner peripheral surface of the fixing belt and presses the fixing belt toward the pressure roll. and the unfixed toner image on the recording medium passing between the pressure roll and the fixing belt to a fixing device according to pressure and heat to the fixed image, the pressure roll of the pressing member The portion pressed against the elastic member is a single member made of a material having an elastic coefficient that causes deformation of the elastic layer when pressed against the elastic layer of the pressure roll and maintains the shape of the pressing member. is formed, the fixing belt of the pressing member The contact surface with the peripheral surface has a curvature in the same direction as the peripheral surface of the pressure roll at the upstream portion in the moving direction of the fixing belt, and the contact pressure increases from the upstream side toward the downstream side in this portion. The fixing device is characterized in that the curvature is smoothly and continuously reversed in the downstream portion, and has a larger curvature than that of the upstream portion in the direction opposite to the peripheral surface of the pressure roll.

  In the pressure contact portion between the pressing member and the pressure roll, at the upstream portion in the moving direction of the fixing belt, the fixing belt has a curvature in the same direction as the circumferential surface of the pressure roll, and the contact pressure increases from the upstream side to the downstream side. Further, it is pressed against the elastic layer of the pressure roll by the pressing member. As a result, a large contact pressure is applied to the recording medium fed between the fixing belt and the pressure roll as the heating progresses, so that good fixing can be performed. At the downstream portion in the moving direction of the fixing belt, the fixing belt is pressed by the pressing member, and the curvature is smoothly reversed continuously. Then, it is pressed against the elastic layer of the pressure roll with the strongest pressing force at this position, and is released from this state at the outlet of the pressing part, so that it moves with a large curvature in the circumferential direction. On the other hand, the recording medium tends to move in the direction when it is ejected from the pressure contact portion due to its own rigidity, and thus is favorably peeled from the fixing belt.

In addition, since the pressing member is made of a material having a sufficiently higher elastic modulus than the elastic layer of the pressure roll, even if it is strongly pressed against the pressure roll, compression set hardly occurs, and the fixing belt is deformed in the moving direction. Hardly occur. For this reason, the shape of the press contact portion between the pressing member and the pressure roll is appropriately maintained over a long period of time, and fixing and peeling of the recording medium can be performed satisfactorily.
The fact that the pressing member is made of a material having a sufficiently higher elastic coefficient than the elastic layer of the pressure roll means that the elastic layer is deformed when the pressing member is pressed against the pressure roll, and the pressure member is ignored. This means that only as much deformation as possible can occur.

  According to a second aspect of the present invention, in the fixing device according to the first aspect, the surface of the pressing member that contacts the inner peripheral surface of the fixing belt is larger in the downstream portion in the moving direction of the fixing belt than in the upstream portion. It shall have curvature.

  The fixing belt is pressed against the elastic layer of the pressure roll by the pressing member at a downstream portion in the moving direction of the fixing belt with a larger curvature than the upstream portion. For this reason, the fixing belt is bent in a direction away from the pressure roll with a large curvature, and in the vicinity of the downstream end of the pressing member, the fixing belt moves with a large curvature in the circumferential direction without being restrained by the pressing member. The medium is discharged in a direction away from the fixing belt.

  According to a third aspect of the present invention, in the fixing device according to the first or second aspect, the surface of the pressing member that contacts the inner peripheral surface of the fixing belt is the center of the contact range in the moving direction of the fixing belt. It is assumed that the curvature is inverted further downstream.

  With the configuration as described above, a portion where the contact pressure of the pressing member against the pressure roll is large is limited to a small range. As a result, the amount of elastic deformation of the elastic layer of the pressure roll is reduced, and the rise of the elastic layer in the vicinity of the press contact portion of the pressing member is reduced. Therefore, it is possible to prevent the recording medium from being pressed against the fixing belt by the raised portion of the elastic layer on the downstream side of the press contact portion, and it is possible to peel the recording medium from the fixing belt satisfactorily. On the other hand, when the curvature of the contact surface between the pressing member and the fixing belt is reversed upstream from the center of the contact range in the moving direction of the fixing belt, the elastic layer of the pressure roll is By elastically deforming in the direction, the elastic layer in the vicinity of the pressure contact portion of the pressing member is greatly raised. Then, the recording medium is pressed against the fixing belt by this portion.

  Further, in the configuration as described above, the contact pressure with respect to the pressure roll by the pressing member is maximized downstream of the center of the contact range in the moving direction of the fixing belt. For this reason, the recording medium and the toner image are gradually heated and pressurized on the upstream side from this position, and the toner image is strongly pressed against the recording medium at the position where the contact pressure is maximized, whereby a good image can be obtained.

  According to a fourth aspect of the present invention, in the fixing device according to the first, second, or third aspect, particles having low thermal conductivity are dispersed in the material forming the pressing member. .

In the pressing member, since the thermal conductivity of the entire material is low due to the dispersion of particles having low thermal conductivity, the amount of heat transferred from the fixing belt to the pressing member is reduced. For this reason, the warm-up time for heating the fixing belt to a predetermined temperature can be shortened. Further, when the fixing operation is continuously performed, it is possible to reduce the temperature of the fixing belt from being lowered, and it is not necessary to frequently perform temperature compensation, so that the production efficiency can be improved.
On the other hand, when the pressing member is an elastic member, if such particles are dispersed, the compression set becomes large due to the pressure contact force with the pressure roll, and the peeling performance may deteriorate over time. Since the pressing member is made of a material having a sufficiently high elastic modulus, even if the particles are dispersed, compression set hardly occurs, and good peeling performance can be maintained over a long period of time.

  According to a fifth aspect of the present invention, in the fixing device according to the first, second, or third aspect, the dispersion amount of the particles having low thermal conductivity is small size recording in the width direction of the fixing belt. It is assumed that the number is set to be large in the paper passing area when the medium is used and small in the non-paper passing area.

  In general, when a small-size recording medium is used, a large amount of heat is taken away by the recording medium and toner in the sheet passing area. For this reason, if the temperature of the fixing belt is controlled to be appropriate in the paper passing area, the temperature of the fixing belt may be excessively higher than that in the paper passing area in the non-paper passing area. However, in the fixing device, the non-sheet passing area of the pressing member has a higher thermal conductivity than the sheet passing area, and the amount of heat dissipated and the amount of heat transmitted to and dissipated by the member supporting the pressing member is increased. . Therefore, the temperature rise in the non-sheet passing area is suppressed, and the temperature difference from the sheet passing area is reduced.

  As described above, according to the present invention, the pressing member pressed against the pressure roll via the fixing belt is made of a material having a sufficiently higher elastic coefficient than the elastic layer of the pressure roll, and strongly pressed against the pressure roll. Even if it is applied, compression set hardly occurs, and deformation of the fixing belt in the moving direction hardly occurs. The contact surface of the pressing member with the inner peripheral surface of the fixing belt has a curvature in the same direction as the peripheral surface of the pressure roll at the upstream portion in the moving direction of the fixing belt, and this portion is downstream from the upstream side. It is set so that the contact pressure increases toward. As a result, a large contact pressure is applied to the recording medium fed between the fixing belt and the pressure roll as the heating progresses, so that good fixing can be performed. The contact surface smoothly and continuously reverses in the downstream portion, and has a curvature in the direction opposite to the circumferential surface of the pressure roll. By being pressed against the layer and released from this state at the outlet of the press contact portion, the layer moves with a large curvature in the circumferential direction. On the other hand, the recording medium moves in the direction when it is discharged from the press contact portion due to its own rigidity, and is favorably peeled off from the fixing belt.

Hereinafter, embodiments of the invention according to the present application will be described with reference to the drawings.
1 and 2 are a schematic sectional view and a schematic front view of a fixing device according to an embodiment of the present invention.
The fixing device includes a fixing belt 1 having an endless peripheral surface, a pressure roll 2 that is in contact with the outer peripheral surface of the fixing belt 1, and an inner peripheral surface of the fixing belt 1. A pressing pad 3 that presses the pressing roll 2, a pad support member 4 that supports the pressing pad, and an electromagnetic induction heating device 5 that is provided along the outer peripheral surface of the fixing belt 1 and heats the fixing belt 1. The peripheral surface of the pressure roll 2 on the downstream side of the pressure contact portion between the guide members 6 a and 6 b that are in contact with the inner peripheral surfaces of both side edges of the fixing belt 1 and the pressure roll 2 and the pressure pad 3. And a peeling auxiliary member 8 provided in the vicinity of the outlet of the pressure contact portion.

  The fixing belt 1 is sandwiched between the pressure roll 2 and the pressure pad 3 and is guided by guide members 6a and 6b at both side edge portions. It is supported. As shown in FIG. 3, the fixing belt 1 includes a base layer 1a made of a sheet member having high heat resistance, a conductive layer 1b laminated thereon, and an elastic layer laminated thereon. It is comprised from the layer 1c and the surface mold release layer 1d used as the uppermost layer. Further, a primer layer may be provided between the layers for adhesion.

  As the base material layer 1, for example, a resin having high heat resistance having a thickness of 10 to 150 μm, more preferably 50 to 100 μm (for example, 75 μm) is used. In this embodiment, a polyimide resin having a thickness of 80 μm is used. ing.

  The conductive layer 1b is a layer that generates heat by electromagnetic induction action of a magnetic field generated by the electromagnetic induction heating device 5, and a metal layer of iron, cobalt, nickel, copper, aluminum, chrome, etc. with a thickness of about 1 to 80 μm. The formed material is used, and the material is selected in consideration of the specific resistance value so that sufficient heat generation can be obtained by eddy current due to electromagnetic induction. In the eddy current according to the present embodiment, copper having a thickness of about 5 μm is used.

  The elastic layer 1c is made of silicone rubber, fluororubber, fluorosilicone rubber or the like having a thickness of 10 to 500 μm, more preferably 50 to 500 μm, and good heat resistance and thermal conductivity. In this embodiment, a silicone rubber having a rubber strength of 15 ° (JIS-A) and a thickness of 300 μm is used.

  The surface release layer 1d is a layer that is in direct contact with the unfixed toner image transferred onto the recording paper, and therefore, it is necessary to use a material having good release properties. Examples of the material constituting the surface release layer 1c include tetrafluoroethylene perfluoroalkyl vinyl ether polymer (PFA), polytetrafluoroethylene (PTFE), silicone resin, silicone rubber, fluorine having a thickness of 10 μm to 50 μm. Rubber etc. are mentioned. In this embodiment, PFA having a thickness of 30 μm is used.

   The pressure roll 2 has a metal cylindrical cored bar 2a as a core, a heat-resistant elastic layer 2b such as silicone rubber or fluororubber on the surface of the cored bar 2a, and a surface release layer on the outermost surface. 2c, and the fixing belt 1 is held between the pressure roll 2 and the pressing pad 3 to form a fixing nip. In this embodiment, the pressure roll 2 has a 5 mm thick sponge layer formed on an iron roll having a diameter of Φ18, and a PFA having a thickness of 50 μm as the uppermost surface release layer on the outer side. It is formed and has a diameter of Φ28.1 and a surface hardness of 60 ° (Asker-C 9.8N).

  The pressure roll 2 is pressed against the pressing pad 3 via the fixing belt 1 with a load of 30 kgf, and is driven to rotate by a drive motor (not shown), whereby the fixing belt 1 is driven to rotate. Then, by passing the recording paper on which the unfixed toner image is transferred to the fixing nip, the unfixed toner image is fixed on the recording paper with heat and pressure supplied from the fixing belt 1 to form a fixed image. It is like that.

The pressing pad 3 is made of a material having a sufficiently higher elastic coefficient than the elastic layer 2b of the pressure roll 2 and presses the fixing belt 1 against the pressure roll 2. The elastic layer 2b of the pressure roll 2 is deformed. Thus, a nip portion is formed between the pressing pad 3 and the pressure roll 2.
As shown in FIG. 4, the contact surface 3 a of the pressing pad 3 with the inner peripheral surface of the fixing belt 1 has a curvature in the same direction as the peripheral surface of the pressure roll 2 in the upstream portion A in the moving direction of the fixing belt 1. However, this portion is set so that the contact pressure increases from the upstream side toward the downstream side. In this portion, a large contact pressure is applied to the recording paper fed between the fixing belt 1 and the pressure roll 2 as the heating progresses.

The curvature of the contact surface 3a is reversed so that it smoothly continues from the upstream portion A at a position C downstream of the center of the contact range in the moving direction of the fixing belt 1. It has a larger curvature than the upstream portion A in the direction opposite to the peripheral surface. In this embodiment, the radius of curvature R _A of the upstream portion A of the contact surface 3a is 60 mm, the radius of curvature R _B of the downstream portion B has a 2 mm. Further, the width is 9 mm, and the curvature is reversed at a position about 8 mm from the nip entrance side. The fixing belt 1 is pressed against the elastic layer of the pressure roll 2 with the strongest pressing force on the downstream side of the position C, and is released from this state in the vicinity of the outlet of the pressing portion. At this time, since the curvature of the downstream portion B of the pressing pad 3 is sufficiently large, the fixing belt 1 moves with a large curvature in the circumferential direction without being restrained by the pressing pad.

  Moreover, since the curvature of the contact surface 3a is reversed in the downstream part B, the part with a large contact pressure with respect to the pressurization roll 2 by the press pad 3 can be kept in a small range. As a result, the amount of elastic deformation of the elastic layer 2b of the pressure roll 2 in the lateral direction is reduced, and the rise of the elastic layer 2b in the vicinity of the pressure contact portion of the pressing pad 3 is reduced. Accordingly, when the recording paper is discharged from the nip portion, it can be prevented from being pressed against the fixing belt 1 by the raised portion of the elastic layer 2b, and the recording paper is peeled off from the fixing belt 1 by its own rigidity. .

  As the material of the pressing pad 3, a metal such as aluminum or iron can be used since it has a large elastic coefficient, but a material having a low thermal conductivity is desirable. PES (polyethersulfone), glass-filled PPS (polyphenylene) Sulfide), LCP (liquid crystal polymer) and the like can be suitably used. By using a material having a high elastic coefficient in this way, the material is hardly deformed when pressed against the pressure roll 2, and permanent deformation is applied to the material even if a frictional force acts in the circumferential direction due to repeated rotation of the pressure roll 2. It is hard to occur. For this reason, the shape of the press-contact part of the press pad 3 and the pressure roll 2 and the press-contact force are appropriately maintained over a long period of time. In this embodiment, PES is used as the material of the pressing pad 3, and the flexural modulus (ASTM D 790) is 2600 MPa.

  In addition, particles having a lower thermal conductivity than the material are dispersed in the material forming the pressing pad 3, and the thermal conductivity of the entire material is reduced by the particles. Thereby, the amount of heat transmitted from the fixing belt 1 can be reduced. As the particles, those having an outer shell made of an inorganic compound such as silica, glass, and alumina and containing air are preferable. If the particle size is too small, the effect cannot be obtained. Since it worsens, about 1 micrometer-200 micrometers are preferable. In this embodiment, hollow glass particles having a diameter of about 10 to 50 μm are dispersed almost uniformly over the axial direction of the pressing pad 3 by 20% by weight. The particles may be mixed in advance into resin pellets as a material, or may be mixed during injection molding or extrusion molding.

  Further, a glass fiber sheet or the like impregnated with a fluororesin having good slidability and high wear resistance may be interposed between the pressing pad 3 and the fixing belt 1, and silicone on the inner surface of the fixing belt. A release agent such as oil may be applied. As a result, the fixing belt 1 follows the pressure roll 2 and circulates at substantially the same speed as the pressure roll 2, thereby reducing the driving load of the pressure roll 2.

The pad support member 4 is a rod-shaped member having an axis in the width direction of the fixing belt 1. Both ends of the rod-shaped member are notched so that the cross section is smaller than the central portion, and the guide member 6 is fitted. The A pressing pad 3 is attached to a portion of the bar-shaped member facing the pressure roll 2, and a pressure contact force acting on the pressing pad 3 from the pressure roll 2 through the fixing belt 1 is borne by the bar-shaped member. As a material for the pad support member 4, a heat-resistant resin such as glass-filled PPS (polyphenylene sulfide), phenol, polyimide, liquid crystal polymer, or heat-resistant glass is used so as not to be heated by the influence of magnetic flux by the electromagnetic induction heating device 5. Can do. Further, a non-magnetic metal having a specific resistance value of 2.7 × 10 ⁻⁸ Ωm or less does not greatly affect the distribution of magnetic flux and hardly generates heat by itself, so that it can be used as a pad support member. Examples of these metals include aluminum, copper, silver, and the like. By using these metals, it is possible to reduce the deflection when a large load is applied to the fixing nip.

  The guide member 6 is made of a heat-resistant resin, and includes a support portion 21 that supports the pad support member 4 and is fixedly supported by the image forming apparatus main body frame, an inner surface guide portion 22, and an edge guide portion 23. .

  The electromagnetic induction heating device 5 is arranged along the outer peripheral surface of the fixing belt 1 so that the gap with the outer peripheral surface is about 2 mm, and heats the conductive layer of the fixing belt 1. The electromagnetic induction heating device 5 includes a pedestal 5a having a curved surface along the fixing belt 1, an excitation coil 5b supported by the pedestal 5a, and an excitation circuit 5c for supplying an alternating current to the excitation coil 5b. It is configured.

When an alternating current is supplied from the excitation circuit 5c to the excitation coil 5b, the magnetic flux repeatedly generates and disappears around the excitation coil 5b. The frequency of the alternating current is set to, for example, 10 to 50 kHz. In this embodiment, the frequency of the alternating current is set to 30 kHz. When this magnetic flux crosses the conductive layer 1b of the fixing belt 1, an eddy current is generated in the conductive layer so as to generate a magnetic field that hinders the change of the magnetic field, and power proportional to the skin resistance of the conductive layer 1b. Joule heat is generated at (W = I ² R). Thereby, the fixing belt 1 is heated to a predetermined temperature.

  The metal roll 7 is made of a metal having good thermal conductivity. In the present embodiment, a solid roll made of aluminum and having a diameter of 10 mm is used. Further, a release layer such as PFA may be provided on the surface of the metal roll 7 so that the toner or the like is less likely to adhere to the surface of the metal roll 7. If the recording paper that is short in the width direction is continuously passed, the temperature of the non-sheet passing area of the fixing belt rises. As a result, the temperature of the non-sheet passing region of the pressure roll 2 rises, so that the metal roll 7 is brought into contact with the peripheral surface of the pressure roll 2 and heat exchange is performed between the pressure roll 2 and the pressure roll 2. A large temperature difference in the width direction of the roll 2 is prevented, and a temperature rise in the non-sheet passing area of the fixing belt is prevented.

  In the peeling assisting member 8, a thin plate member such as stainless steel is used as a substrate 8a, and an elastic member 8b such as silicone rubber or fluororubber is provided at the tip of the substrate 8a. The front end portion is disposed so as to face the fixing belt 1 in close proximity at the exit of the nip portion. The peeling assisting member 8 scoops the leading edge of the recording paper that has passed through the nip and peeled off from the fixing belt 1 to prevent the recording paper from being wound around the fixing belt 1 again. Since the elastic member 8b is provided at the tip of the peeling assisting member 8, the fixing belt is hardly damaged even if the tip contacts the fixing belt 1. In this embodiment, a fluororubber coating layer is provided at the tip of a 0.2 mm stainless steel substrate, and the tip is sharpened by providing an inclined surface at an angle of 35 ° with the direction of the substrate. Yes. The horizontal distance from the exit of the nip portion to the front end is 5 mm, and the distance from the fixing belt when the rotation of the fixing belt 1 is stopped is 0.5 mm.

Next, the operation of the fixing device will be described.
A toner image T of four color toners of yellow, magenta, cyan, and black is formed based on the image signal, and transferred to the recording paper P by a transfer device (not shown). These toners are constituted by containing a color pigment in a thermoplastic resin binder.
On the other hand, almost simultaneously with the start of the operation for forming the toner image, power is supplied to the drive motor (not shown) of the pressure roll 2 and the electromagnetic induction heating device 5. Then, when the pressure roll 2 rotates, the fixing belt 1 starts to rotate following this, and an eddy current is induced in the conductive layer 1 b when passing through the heating region facing the electromagnetic induction heating device 5. Fever. At this time, the fixing belt 1 slides in contact with the inner surface guide portion 22 in the vicinity of both side edges, but the central portion moves around without contacting any member other than the pressing pad 3. Therefore, heat is not taken away by other members, and efficient heating is performed.

  When the fixing belt 1 circulates and is heated to a predetermined temperature, the recording paper carrying the unfixed toner image is sent to the nip portion where the fixing belt 1 and the pressure roll 2 are in contact with each other. In the nip portion, the contact pressure by the pressing pad 3 increases from the entrance of the nip portion to the position C where the curvature of the contact surface is reversed, and the recording paper and the toner image are gradually heated and pressurized, and the downstream side of the position C The toner image is pressed against the toner image.

  The fixing belt 1 is released from this state in the vicinity of the exit of the nip portion, and moves with a large curvature in the circumferential direction. On the other hand, the recording paper conveyed between the fixing belt 1 and the pressure roll 2 has a pressure contact force reduced in the vicinity of the exit of the nip portion, and tends to proceed in a direction in which the curvature decreases due to the rigidity of the recording paper itself. Then, it is pulled away from the fixing belt 1. When the leading edge of the recording paper is peeled off from the fixing belt 1, the recording paper comes into contact with the peeling auxiliary member 8 and is guided by the peeling auxiliary member 8 to be surely pulled away from the fixing belt 1.

  In this embodiment, even when a thin recording paper of about 60 gsm is used and a large amount of toners of a plurality of colors are carried behind the 3 mm leading edge margin, the recording paper can be peeled off from the fixing belt satisfactorily.

Here, the recording paper peeling performance over time when the pressing pad 3 made of PES of this embodiment is used and when the pressing pad made of Si rubber, which is a conventional elastic member, is used as a comparison object. Table 1 shows the results of an experiment examining various changes.
In this experiment, during driving of the fixing device of 200 kpv cycle, fixing was performed by placing a solid image on the entire area behind 3 mm of the leading edge margin of 60 gsm A4 recording paper every 20 kpv cycle. Then, the toner amount (g / m ² ) of the solid image portion of the recording paper that could be peeled off without occurrence of recording paper jam or image defect due to peeling failure was measured. That is, when fixing a plurality of sheets by changing the amount of toner of a solid image, a toner with a large amount of toner tends to cause a peeling failure, but the upper limit of the amount of toner that can be satisfactorily peeled is measured. In addition, a general character chart was used as an image while 20 kpv cycle passed.

From Table 1, in the pressing pad 3 of the present embodiment, the toner amount on the peelable recording paper does not change to 15 g / m ² from the initial driving of the apparatus until the driving of the 200 kpv cycle is completed, and the 200 kpv cycle is being driven Further, it can be seen that the recording paper peeling performance by the pressing pad is maintained. On the other hand, with the conventional press pad, the toner amount on the recording paper that can be peeled off at the initial stage of driving is 15 g / m ² , which is the same as that of the press pad 3 of this embodiment, but can be peeled off as the fixing operation progresses. The amount of toner on the recording paper decreased and was 10 g / m ² after driving 200 kpv cycles. Further, it was confirmed that after the driving of 200 kpv cycle, compression set was generated in the rubber as the pressing pad. That is, in the conventional press pad, since the rubber is deformed with time, it is difficult to maintain a stable peeling performance over a long period of time.

On the other hand, in the fixing device of this embodiment, the temperature at the nip entrance is controlled to 180 ° C. at the end of preheating after power-on, and 170 ° C. at the time of normal fixing. A color fixed image can be formed.
In this fixing device, the time required for warming up the fixing belt at the time of startup, that is, the time required to reach 180 ° C. is 8.5 seconds when the power of 1200 W is turned on as shown in FIG. On the other hand, when a conventional pressing pad is used, 9.8 seconds are required. In this embodiment, particles with low thermal conductivity are dispersed in the pressure pad, so that the warm-up time is reduced by 1.3 seconds compared to the case where a conventional pressure pad is used, and the apparatus can be operated with a very short waiting time. It will be ready for use. For this reason, it is sufficient to supply power only when it is used, and it is not necessary to use a standby mode or a low power mode, so that power consumption can be reduced.

Next, a pressing pad that can be used in place of the pressing pad 3 will be described with reference to FIG. The pressing pad is used in the fixing device according to an embodiment of the fifth aspect.
The pressing pad 31 is made of PES having a sufficiently higher elastic coefficient than the elastic layer 2 b of the pressure roll 2, and has the same shape as the pressing pad 3. In the pressing pad 31, particles 32 having a low thermal conductivity similar to the pressing pad 3 are dispersed in the sheet passing area when a small-size recording paper is used in the width direction of the fixing belt 1, and non-conductive. Less distributed in the paper passing area. In other words, since this fixing device performs paper feeding based on the axial center position (center resist), the weight ratio is 20% from the axial center position to the 210 mm area on both sides. The hollow glass particles having a weight ratio of 10% are dispersed at both ends. By dispersing particles with low thermal conductivity in this way, when a small-size recording paper is continuously passed, a large amount of heat is dissipated through the pressing pad in the non-sheet passing area, and from the sheet passing area. Less heat dissipation. Accordingly, the temperature distribution hardly occurs in the width direction of the fixing belt 1.

  In order to verify the effect of the pressing pad 31, 500 sheets of A4 recording paper (210 mm × 297 mm) was passed at a speed of 21 sheets per minute with the short edge at the leading edge, and the fixing belt 1 was not passed. The temperature distribution in the axial direction of the pressing pad 31 when the temperature in the paper area reached the maximum was measured. This result is shown in FIG. 7 in comparison with the case where the pressing pad 3 shown in FIG. 1 is used.

  From FIG. 7, when the pressing pad 31 in which the mixing amount of the particles is changed in the width direction is used, the maximum temperature of the non-sheet passing region of the fixing belt 1 is about 215 ° C., and the pressing pad 3 shown in FIG. Compared to about 225 ° C. when using this, it is about 10 ° C. lower. This is because the pressure pad 31 has a large amount of heat transfer from the non-sheet passing area of the fixing belt 1 and a small amount of heat transfer from the sheet passing area, so that a decrease in the temperature of the sheet passing area is suppressed. This is because the heating time (frequency of temperature compensation) for maintaining the paper passing area at a predetermined temperature is reduced. By reducing the heating time of the fixing belt 1, it is possible to prevent the non-sheet passing area of the fixing belt 1 from being excessively heated and to enable continuous sheet passing, thereby improving production efficiency. .

In the fixing device described above, instead of the pressing pads 3 and 31, for example, pressing pads 41, 42, and 43 having the shapes shown in FIGS. 8, 9, and 10 may be used. Table 2, the contact surface 41a of the pressure roll 2 of the pressure pad 41, 42, 43, 42a, of 43a, the radius of curvature R _An upstream portion An, the curvature radius R _Bn downstream portion Bn, curvature inversion The position Cn to be performed and the width Dn of the pressing pad are shown.

  Even when a pressing pad having such a shape is used, the toner image can be sufficiently melt-bonded to the recording paper in the pressure contact portion with the pressure roll 2 and the recording paper can be peeled off from the fixing belt 1 satisfactorily. . The shape of the pressure pad used in the fixing device is not limited to that shown in FIGS. 8, 9, and 10. If the shape of the pressure pad shown in this embodiment satisfies the conditions, the fixing is performed. It can be appropriately selected depending on the relationship with other members of the apparatus, required peeling performance and productivity.

1 is a schematic cross-sectional view of a fixing device according to an embodiment of the present invention. 1 is a schematic front view of a fixing device according to an embodiment of the present invention. FIG. 2 is an enlarged sectional view showing a fixing belt used in the fixing device shown in FIG. 1. FIG. 2 is an enlarged view of a pressure contact portion between a pressure pad and a pressure roll used in the fixing device shown in FIG. 1. It is a figure which compares the warm-up time at the time of using the press pad of a present Example, and the conventional press pad. FIG. 6 is a schematic front view of a fixing belt and a pressure pad used in the fixing device according to an embodiment of the present invention, and a cross-sectional view of the pressure pad. FIG. 4 is a diagram showing a difference in temperature distribution in the width direction of the fixing belt 1 when the distribution of particles dispersed in the pressing pad is different. It is an enlarged shape figure of the press-contact part of the other press pad and pressure roll which can be used with the fixing device shown in FIG. It is an enlarged shape figure of the press-contact part of the other press pad and pressure roll which can be used with the fixing device shown in FIG. It is an enlarged shape figure of the press-contact part of the other press pad and pressure roll which can be used with the fixing device shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1: Fixing belt, 2: Pressure roll, 3, 31, 41, 42, 43: Press pad, 4: Pad support member, 5: Electromagnetic induction heating apparatus, 5a: Base, 5b: Excitation coil, 5c: Excitation circuit 6: Guide member, 7: Metal roll, 8: Peeling auxiliary member, 21: Support part, 22: Inner surface guide part, 23: Edge guide part

Claims (5)

An endless fixing belt in an unstretched state supported so as to be capable of circular movement;
A heating device for heating the fixing belt;
A pressure roll that is arranged to face the outer peripheral surface of the fixing belt and includes an elastic layer in the vicinity of the peripheral surface;
A pressing member that is in contact with the inner peripheral surface of the fixing belt and presses the fixing belt against the pressure roll side;
A fixing device that pressurizes and heats an unfixed toner image on a recording medium passing between the fixing belt and the pressure roll to form a fixed image;
The portion of the pressing member that presses against the pressure roll causes the elastic layer to be deformed when pressed against the elastic layer of the pressure roll, so that the shape of the pressing member is maintained. Formed of a single member made of
The contact surface of the pressing member with the inner peripheral surface of the fixing belt has a curvature in the same direction as the peripheral surface of the pressure roll at the upstream portion in the moving direction of the fixing belt, and this portion is downstream from the upstream side. It is set so that the contact pressure increases toward the side, the curvature is smoothly and continuously reversed in the downstream portion, and has a larger curvature than the upstream portion in the direction opposite to the peripheral surface of the pressure roll. Fixing device.   The fixing device according to claim 1, wherein a surface of the pressing member that is in contact with an inner peripheral surface of the fixing belt has a curvature larger than that of an upstream portion in a downstream portion in a moving direction of the fixing belt.   The curvature of the surface of the pressing member that contacts the inner peripheral surface of the fixing belt is inverted downstream from the center of the contact range in the moving direction of the fixing belt. The fixing device according to 1.   The fixing device according to claim 1, wherein particles having low thermal conductivity are dispersed in a material forming the pressing member. The dispersion amount of the particles having low thermal conductivity is set to be large in a width direction of the fixing belt in a sheet passing area when a small-size recording medium is used and small in a non-sheet passing area. The fixing device according to claim 1, claim 2 or claim 3.

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