JP2005234103A - Fixing device - Google Patents

Abstract

An object of the present invention is to improve the peelability of a recording medium from a fixing belt and improve image quality.
A fixing roller, a stretching roller, an endless fixing belt that is stretched between the fixing roller and the stretching roller, and a fixing belt that is disposed outside the fixing belt. A pressure roller 12 that is in pressure contact with the roller 11 and a pressure roller 12 that is disposed inside the fixing belt 16 and that is downstream of the pressure contact portion where the pressure roller 12 is in pressure contact with the fixing roller 11. And a pressing member. In this case, since the pressing member is disposed on the downstream side of the pressure contact portion where the pressure roller 12 is brought into pressure contact with the fixing roller 11, the nip width can be increased, and the fixing belt 16 on the outlet side of the nip portion can be increased. The curvature can be increased.
[Selection] Figure 1

Description

  The present invention relates to a fixing device.

  2. Description of the Related Art Conventionally, image forming apparatuses such as printers, copiers, and facsimile machines, for example, electrophotographic printers, are provided with a fixing device for fixing a toner image transferred onto a sheet as a recording medium. It has become. The fixing device includes a fixing roller and a pressure roller, and the sheet passes through a nip portion formed between the fixing roller and the pressure roller, and a toner image is fixed on the sheet during that time (for example, , See Patent Document 1).

  FIG. 2 is a cross-sectional view of a conventional fixing device.

  In the figure, reference numeral 11 denotes a fixing roller that includes a heat source 13 and has an elastic layer 11b formed on the outer side of the element tube 11a and is rotated by driving a motor (not shown), and 12 is brought into pressure contact with the fixing roller 11. It is a pressure roller that is disposed and has a built-in heat source 14, an elastic layer 12 b is formed on the surface of the raw tube 12 a, and is rotated by being driven by the fixing roller 11.

  The sheet 15 on which the toner image has been transferred passes through a nip formed between the fixing roller 11 and the pressure roller 12, and the toner image is fixed on the sheet 15 during that time.

  Incidentally, a belt-type fixing device has been proposed in order to increase the fixing efficiency of the fixing device (see, for example, Patent Document 2).

  FIG. 3 is a cross-sectional view of a conventional belt-type fixing device.

In this case, the fixing belt 16 is stretched between the fixing roller 11 and the stretching roller 17, and the fixing belt 16 is caused to travel in the arrow direction as the fixing roller 11 rotates. Further, the pressure roller 12 is pressed against the fixing roller 11 via the fixing belt 16, and a nip portion is formed between the fixing belt 16 and the pressure roller 12. Since the hardness of the elastic layer 12b on the pressure roller 12 side is formed lower than the hardness of the elastic layer 11b on the fixing roller 11 side, the pressure roller 12 is brought into pressure contact with the fixing roller 11 at the nip portion, thereby causing the elasticity. The layer 12b is deformed according to the shape of the fixing roller 11 to form a concave surface. A contact area where the fixing belt 16 and the pressure roller 12 are in contact with each other with a low pressure is formed on the upstream side of the nip portion in the running direction of the fixing belt 16. The contact area functions as a heating area, and the heat generated by the heat sources 13 and 14 is transmitted to the fixing belt 16 via the fixing roller 11 and the pressure roller 12 and further to the toner image on the paper 15. Is done. Therefore, the fixing efficiency of the fixing device can be increased.
Japanese Patent Laid-Open No. 5-232828 JP 2002-258661 A

  However, in the conventional fixing device, as described above, since the concave surface is formed on the pressure roller 12 side in the nip portion, the curvature on the downstream side of the nip portion, that is, the outlet side of the nip portion is fixed. It depends on the diameter of the roller 11. Accordingly, when the diameter becomes larger than a predetermined value and the curvature on the side facing the toner image becomes small, it becomes impossible to obtain a force for separating the paper 15 from the fixing belt 16, and the paper 15 is fixed to the fixing belt. 16 will wrap around. In particular, when thin paper with low stiffness is used as the paper 15 or solid printing with a large amount of toner used is performed at high speed, when the diameter of the fixing roller 11 is 20 mm or more, the paper 15 becomes the fixing belt 16. Experience has shown that there are many cases of wrapping around. Then, when the paper 15 is wound around the fixing belt 16, the toner of the toner image is disturbed, and the image quality is deteriorated.

  An object of the present invention is to provide a fixing device that solves the problems of the conventional fixing device, improves the peelability of the recording medium from the fixing belt, and improves the image quality.

  Therefore, in the fixing device of the present invention, the fixing roller, a tension roller disposed in parallel with the fixing roller, and an endless fixing belt stretched between the fixing roller and the tension roller. A pressure roller disposed outside the fixing belt and pressed against the fixing roller via the fixing belt, and a pressure roller disposed inside the fixing belt and in the recording medium conveyance direction. And a pressing member for pressing the pressure roller via the fixing belt on the downstream side of the pressure contact portion where the pressure roller is pressed against the fixing roller.

  As described above in detail, according to the present invention, in the fixing device, the fixing roller, the stretching roller disposed in parallel to the fixing roller, and the tensioning roller between the fixing roller and the stretching roller. An endless fixing belt provided; a pressure roller disposed outside the fixing belt and pressed against the fixing roller via the fixing belt; and a recording medium disposed inside the fixing belt. And a pressing member for pressing the pressure roller via the fixing belt on the downstream side of the pressure contact portion where the pressure roller is pressed against the fixing roller.

  In this case, since the pressing member is disposed downstream of the pressure contact portion where the pressure roller is pressed against the fixing roller, the nip width can be increased and the curvature of the fixing belt on the outlet side of the nip portion can be increased. Can be bigger. Accordingly, since the peelability of the recording medium from the fixing belt can be improved, the recording medium can be prevented from being wound around the fixing belt, and the image quality can be improved.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In this case, a fixing device applied to an electrophotographic printer as an image forming apparatus will be described.

  In the electrophotographic printer, a toner image as a developer image is transferred to a paper as a recording medium in an image forming unit, the paper is sent to a fixing device, and the toner image is fixed on the paper in the fixing device. Be made. In this embodiment, paper is used as a recording medium, but an OHP sheet, an envelope, or the like can be used instead of the paper.

  The image forming unit includes a photosensitive drum as an image carrier, a charging roller as a charging device, an exposure device, a developing device, a transfer roller as a transfer device, and the like. The surface of the photosensitive drum is uniformly and uniformly charged by the charging roller, and the surface of the photosensitive drum is exposed by the exposure device to form an electrostatic latent image. Subsequently, the electrostatic latent image is developed by a developing device to form a toner image, and the toner image is transferred to a sheet by a transfer roller.

  FIG. 1 is a cross-sectional view of a fixing device according to a first embodiment of the present invention.

  In the figure, reference numeral 11 denotes a fixing roller as a first roller incorporating a heat source 13 composed of a halogen heater or the like, and the fixing roller 11 is rotated by driving a motor as a driving unit (not shown). Reference numeral 12 denotes a pressure roller as a second roller which is disposed in pressure contact with the fixing roller 11 and incorporates a heat source 14 composed of a halogen heater or the like. The pressure roller 12 is a fixing roller. 11 to be rotated. The pressure roller 12 is urged toward the fixing roller 11 by a spring 21 as a first urging member, and the urging force of the spring 21 is applied to both ends of a shaft (not shown) of the pressure roller 12. .

  In this embodiment, the pressure roller 12 is rotated by rotating the fixing roller 11. However, both the fixing roller 11 and the pressure roller 12 are rotated or added by the motor. Only the pressure roller 12 can be rotated by the motor, and the fixing roller 11 can be driven to rotate by the pressure roller 12.

  A tension roller 17 is a third roller disposed at a predetermined distance from the fixing roller 11 and in parallel with the fixing roller 11 and the pressure roller 12, and 20 is a pressing member. An endless fixing belt 16 as a fixing member is stretched between the stretching roller 17 and the fixing roller 11. The pressure roller 12 is disposed outside the fixing belt 16, whereas the tension roller 17, the pressure pad 20, and the fixing roller 11 are all disposed inside the fixing belt 16 and are illustrated in the drawing. It is supported by a predetermined support member that is not. The tension roller 17 is biased in the direction opposite to the fixing roller 11 by a spring 22 as a second biasing member, and the fixing belt 16 is stretched. At this time, the urging force of the spring 22 is applied to both ends of a shaft (not shown) of the stretching roller 17 to apply tension to the fixing belt 16. Further, a thermistor 23 serving as a temperature detecting unit for detecting the surface temperature of the fixing belt 16 is disposed in contact with the surface of the fixing belt 16 and opposed to the fixing roller 11, and is detected. The temperature is sent to a control unit (not shown).

  By the way, the pressure roller 12 is pressed against the fixing roller 11 and the pressure pad 20 via the fixing belt 16 by the urging force of the spring 21, and the first pressure contact is made between the pressure roller 12 and the fixing roller 11. A second pressure contact portion is formed between the pressure roller 12 and the pressure pad 20. At this time, the urging force by the spring 21 becomes the first and second pressing forces against the fixing roller 11 and the pressure pad 20. When the pressure roller 12 is brought into pressure contact with the fixing roller 11 and the pressure pad 20, a nip portion is formed between the pressure roller 12 and the fixing belt 16 by the fixing roller 11, the pressure pad 20 and the pressure roller 12. Is formed. In the nip portion, as shown in the drawing, as the pressure roller 12 is brought into pressure contact with the fixing roller 11, the pressure roller 12 is pushed in by the fixing roller 11, and has a concave shape.

  The pressure pad 20 is disposed downstream of the first pressure contact portion in the conveyance direction of the paper 15, that is, on the outlet side of the first pressure contact portion, and presses the pressure roller 12 through the fixing belt 16. . The pressure pad 20 contacts the fixing belt 16 and slides, and the portion that contacts the pressure roller 12 via the fixing belt 16, that is, the contact portion 20a has an R shape. Then, on the exit side of the nip portion, the curvature of the fixing belt 16 on the side facing the toner image is reduced. Accordingly, the fixing belt 16 is deflected toward the pressure pad 20 on the downstream side of the second pressure contact portion in the conveyance direction of the sheet 15, that is, on the outlet side of the second pressure contact portion. The releasability of the paper 15 as a medium can be improved, the nip width can be widened, and the fixing performance can be improved.

  Next, the dimensions and materials of each element will be described. In this embodiment, the fixing belt 16 corresponding to the A4 size paper 15 is used.

  The fixing roller 11 is made of an aluminum base tube having an outer diameter of 30 [mm] and a thickness of 2 [mm]. The pressure roller 12 has an outer diameter of 40 [mm] and a thickness of 1 [on the outside of an aluminum base tube 12a constituting a core metal having a thickness of 2 [mm]. mm] of silicone rubber is formed. A fluorine-based coat layer (not shown) having a thickness of about 0.03 to 0.05 [mm] is formed outside the elastic layer 12b.

  The tension roller 17 has an outer diameter of 16 [mm], and is formed of a heat-insulating and heat-resistant material having a thickness of 1 [mm] on the outside of a solid iron core, for example, silicone rubber. A sponge layer is formed.

  The pressure pad 20 is made of a highly rigid iron-based metal, and a surface treatment for reducing the frictional force, for example, coating with a fluorine-based resin, is applied to a sliding portion with the fixing belt 16. The fixing belt 16 has a thickness on the outside of a base material made of a polyimide film having a thickness of about 0.07 [mm] or a metal such as nickel or stainless steel having a thickness of about 0.04 [mm]. A heat-resistant elastic layer made of 0.2 to 0.3 [mm] silicone rubber or the like is formed, and on the outer side, the thickness is about 0.03 to 0.05 [mm] and has good releasability A fluorine-based coat layer is formed.

  Further, the nip width is set to about 10 [mm], and the radius of the R shape of the contact portion 20a is the upper limit in order to increase the curvature of the fixing belt 16 on the side facing the toner image on the exit side of the nip portion. Set to below value. In this case, the upper limit value is a value at which the paper 15 does not wrap around the fixing belt 16 even when thin paper with a low stiffness is used or solid printing with a large amount of toner as a developer is performed at high speed. It is calculated | required by experiment and is set to 10 [mm] in this Embodiment.

  As for the upper limit of the radius of the R shape, as described above, when the diameter of the fixing roller 11 is 20 [mm] or more, thin paper with a low stiffness or a large amount of toner is used. When printing is performed at a high speed, the paper 15 is preferably wound around the fixing belt 16 so that the thickness is preferably 10 [mm].

  In the fixing device configured as described above, when a printing command is issued from the control unit, the heat sources 13 and 14 are energized, the fixing roller 11 and the pressure roller 12 are heated, and the motor is driven to rotate the fixing roller 11. Then, the pressure roller 12 is driven and rotated, and the fixing belt 16 is caused to travel.

  When the surface temperature of the fixing belt 16 detected by the thermistor 23 reaches a set value, the sheet 15 to which the unfixed toner image conveyed from the image forming portion is transferred passes through the nip portion, and the toner image is It is fixed on the paper 15. Subsequently, the sheet 15 is peeled off from the fixing belt 16 on the exit side of the nip portion and discharged.

  Thus, in the present embodiment, the pressure pad 20 is disposed on the outlet side of the first pressure contact portion inside the fixing belt 16, so that the nip width can be increased and the contact portion 20a. Since the radius of the R shape is reduced, the curvature of the fixing belt 16 on the exit side of the nip portion can be increased. Therefore, the releasability of the paper 15 from the fixing belt 16 can be improved, and the force for peeling the paper 15 from the fixing belt 16 can be increased. As a result, it is possible to prevent the paper 15 from being wound around the fixing belt 16, and accordingly, the toner of the toner image does not adhere to the fixing belt 16 and is not disturbed, so that the image quality can be improved. .

  Further, since it is not necessary to reduce the diameter of the fixing roller 11, it is possible to prevent the bending rigidity of the fixing roller 11 from being lowered. Therefore, the fixing roller 11 is not bent by the first pressing force, the tension of the fixing belt 16, and the like, and the nip width and the first pressing force can be made uniform in the longitudinal direction of the fixing roller 11. The quality can be improved.

  Further, since it is not necessary to provide a separation claw on the exit side of the nip portion, the surface of the fixing belt 16 is not damaged. Therefore, the image quality can be improved.

  Next, a second embodiment of the present invention will be described. In addition, about the thing which has the same structure as 1st Embodiment, the description is abbreviate | omitted by providing the same code | symbol, and the effect of the same embodiment is used about the effect of the invention by having the same structure. .

  FIG. 4 is a cross-sectional view of the fixing device according to the second embodiment of the present invention, FIG. 5 is a diagram showing the pressure distribution in the nip portion according to the first embodiment of the present invention, and FIG. 6 is the second embodiment of the present invention. It is a figure which shows the pressure distribution of the nip part in embodiment.

  In this case, the first and second pressing forces f1 and f2 in the present embodiment are different from the first and second pressing forces g1 and g2 in the first embodiment.

  In other words, in the first embodiment, the first pressing force g1 (for example, 8 [kgf]) in the first pressing portion is changed to the second pressing force g2 (for example, 5 [kgf] in the second pressing portion). ]) And a pressure distribution as shown in FIG. 5 is formed in the nip portion. On the other hand, in the present embodiment, the first pressing force f1 (for example, 4 [kgf]) at the first pressing portion is changed to the second pressing force f2 (for example, 8 [for example, 8 [ kgf]), and a pressure distribution as shown in FIG. 6 is formed in the nip portion.

  That is, in the pressure distribution shown in FIG. 5, the maximum value of the first pressing force g1 is formed in the pressure distribution region q1 of the pressure distribution regions q1 and q2 on the fixing roller 11 side and the pressure pad 20 side. In the pressure distribution shown in FIG. 6, the maximum value of the second pressing force f2 is formed in the pressure distribution region p2 of the pressure distribution regions p1 and p2 on the fixing roller 11 side and the pressure pad 20 side. In this embodiment, the pressure center of gravity of the pressure distribution in the nip portion is moved downstream in the conveyance direction of the sheet 15 and the traveling direction of the fixing belt 16.

  For this purpose, the positions of the fixing roller 11 and the pressure pad 20 with respect to the pressure roller 12 are set. A spring as a third urging member is provided to urge the pressure pad 20 toward the pressure roller 12 with a predetermined urging force in correspondence with necessity, and the second pressing force f2 is set. Can be bigger. In the present embodiment, the thickness of the base tube of the fixing roller 11 is set to 1 [mm].

  By the way, in the fixing device, the toner image as the developer image on the paper 15 as the recording medium is heated and softened as it enters the nip portion, but an appropriate pressure is required to obtain sufficient fixing property. Is required. That is, the fixing property can be improved by causing the toner softened and melted on the surface of the paper 15 to adhere well to the paper 15, but an appropriate pressure is required for this purpose.

In this case, in the first half of the sheet 15 passing through the nip portion, it is not necessary to increase the pressure, and the fixing belt 16 and the toner can be transferred from the fixing belt 16 as a fixing member to the toner image. It is sufficient if there is enough pressure to ensure contact with the image. In a color fixing device, generally, the maximum pressure at the nip is about 3 [kgf / cm ² ].

  Therefore, as described above, the first pressing force f1 is made smaller than the second pressing force f2, and the pressure is lowered in the first half when the sheet 15 passes through the nip portion, and is increased in the second half. ing.

  Therefore, in this embodiment, even when printing is performed at high speed, fixing can be performed satisfactorily.

  Furthermore, the rigidity of the fixing roller 11 can be reduced by the amount that the first pressing force f1 can be reduced, and the thickness of the raw tube can be reduced. Therefore, the heat responsiveness of the fixing roller 11 can be improved, and the time required for heating the fixing roller 11, that is, the rise time can be shortened.

  In the present embodiment, since the first pressing force f1 is set to ½ of the second pressing force f2, when the fixing roller 11 having an outer diameter of 30 [mm] is used, the fixing roller 11 Thus, the thickness of the metal core with the same amount of deflection 11 can be reduced from 2 [mm] to 0.9 [mm].

  By the way, when the sheet 15 passes through the nip portion and the fixing is completed, the sheet 15 is curled. However, the amount of curling, that is, the curl amount and the direction of curling depends on the shape of the nip section, the pressing force, It is greatly affected by the thickness. For example, in an upper and lower roller type fixing device including only the fixing roller 11 and the pressure roller 12, the sheet 15 is fixed when the cross-sectional shape of the nip portion is a shape that forms a recess on the pressure roller 12 side. The amount of curling increases as the curling to the roller 11 side and the curvature of the fixing roller 11 increases. Also, the greater the pressing force, the greater the curl amount. As the thickness of the sheet 15 is increased, the nip portion has a cross-sectional shape with a tendency to be wrinkled, and the curl amount is increased.

  In each of the above-described embodiments, the pressure pad 20 as the pressing member is disposed. Therefore, the R shape of the abutting portion 20a (FIG. 1) has the most influence on the curl amount, and the radius of the R shape is larger. The curl amount is small, and the curl amount increases as the radius of the R shape decreases.

  Therefore, a third embodiment of the present invention in which the second pressing force f2 is adjusted according to the type of the paper 15 without changing the shape of the pressure pad 20 and the curl amount can be reduced will be described. To do. In addition, about the thing which has the same structure as 2nd Embodiment, the description is abbreviate | omitted by providing the same code | symbol, and the effect of the embodiment is used about the effect of the invention by having the same structure. .

  FIG. 7 is a cross-sectional view of the fixing device according to the third embodiment of the present invention.

  In this case, the pressure pad 20 as the pressing member is supported so as to be movable in the radial direction of the pressure roller 12 as the second roller, and is attached to the pressure roller 12 by the spring 30 as the third biasing member. It is energized with a predetermined energizing force. Therefore, the second pressing force f <b> 2 is configured by the biasing force by the spring 21 as the first biasing member and the biasing force by the spring 30.

  Further, a cam mechanism M as a pressing force adjusting device is disposed behind the spring 30 (left side in the figure). The cam mechanism M includes a cam 31 that is rotatably disposed, and a spring 30. A follower 32 connected to the rear is provided. The cam 31 is positioned at a plurality of positions so that the biasing force of the spring 30 can be changed in accordance with the thickness of the paper 15 as a recording medium. The cam 31 is connected to a positioning motor (not shown) as a positioning drive unit, and the second pressing force f2 is adjusted by driving the positioning motor, rotating the cam 31, and positioning. can do.

  By the way, the printer main body is provided with a setting mechanism for setting a menu for performing printing under optimum printing conditions corresponding to the type of the paper 15, and a detection device for detecting the thickness of the paper 15. The pressing force adjustment processing means of the control unit performs the pressing force adjustment processing, drives the positioning motor based on information from the setting mechanism and the detection device, and corresponds to the type of the paper 15 to adjust the fixing device. Change characteristics.

  Next, the operation of the fixing device having the above configuration will be described.

  When the control unit issues a print command, the controller drives the positioning motor and positions the cam 31 based on the information on the thickness of the paper 15 acquired in advance from the detection device. For example, since the curl amount is small when the paper 15 is thin paper, the second pressing force f2 is set to the default setting state, and when the paper 15 is thick paper, the curl amount increases. , And the cam 31 is rotated to a position where the second pressing force f2 becomes small, and the positioning is performed.

  Thus, in the present embodiment, the second pressing force f2 is adjusted in accordance with the thickness of the paper 15, so that the curl amount can be reduced regardless of the type of the paper 15.

  By the way, the pressure pad 20 always slides with respect to the fixing belt 16 as a fixing member, so that the fixing belt 16 and the pressure pad 20 are worn and the durability of the fixing device is lowered. The torque for running the belt 16, that is, the torque for rotating the fixing roller 11 as the first roller increases, and the reliability of the fixing device over time decreases.

  Therefore, a fourth embodiment of the present invention in which the pressure pad 20 is slid with respect to the fixing belt 16 only when necessary will be described. In addition, about the thing which has the same structure as 2nd Embodiment, the description is abbreviate | omitted by providing the same code | symbol, and the effect of the embodiment is used about the effect of the invention by having the same structure. .

  FIG. 8 is a cross-sectional view showing a state where the pressure pad according to the fourth embodiment of the present invention is placed in the operating position, and FIG. 9 shows a state where the pressure pad according to the fourth embodiment of the present invention is placed in the retracted position. It is sectional drawing shown.

  The pressure pad 20 as the pressing member needs to come into contact with the fixing belt 16 as the fixing member only when the paper 15 as the recording medium passes through the nip portion, that is, when the paper is passing. Place the pressure pad 20 in contact with the fixing belt 16 at the time of paper, that is, the operating position, and place the pressure pad 20 in the position that does not contact the fixing belt 16 at other times, that is, when not passing, that is, in the retracted position. .

  For this purpose, the pressure pad 20 is supported by a support member 25 that is rotatable coaxially with the axis of the fixing roller 11 as the first roller. The support member 25 has an arcuate shape extending over a predetermined angle along the outer peripheral surface of the fixing roller 11 and includes a sliding portion (not shown). The bearing of the fixing roller 11 is fixed to the sliding portion. It is inscribed in a round frame formed on the apparatus main body (not shown) and is rotatably supported. The pressure pad 20 is attached to one end of the support member 25, and a protrusion 25a is formed integrally with the other end so as to protrude radially outward. A spring 27 as a fourth urging member is disposed between the protrusion 25a and the apparatus main body, and the spring 27 causes the protrusion 25a to be attached to the pressure pad 20 by a predetermined urging force. The urging is performed in a direction in which the pressure roller 12 as the second roller is pressed through the fixing belt 16. The urging force of the spring 27 and the urging force of the spring 21 as the first urging member are added to form a second pressing force f2.

  Further, a plunger 26 is disposed so as to face the protruding portion 25a. When the plunger 26 is turned on, a rod 26a as a pressing member of the plunger 26 is moved forward (moved in the right direction in the figure), and the protrusion The portion 25a is pressed against the urging force of the spring 27, and the pressure pad 20 is placed in the retracted position as shown in FIG. When the plunger 26 is turned off, the rod 26a is retracted (moved leftward in the figure), and the protrusion 25a is returned by the urging force of the spring 27, and as shown in FIG. Place the pad 20 in the operating position.

  Next, the operation of the fixing device having the above configuration will be described.

  In this case, when the printer is stopped, the pressing force adjustment processing means turns off the plunger 26 and presses the pressure pad 20 against the fixing belt 16 by the urging force of the spring 27 and puts it in the operating position. When the control unit issues a print command, the heaters 13 and 14 are energized until the surface temperature of the fixing belt 16 reaches a predetermined temperature. During this period, the control unit performs the fixing roller 11 and the pressure roller. 12, the fixing belt 16, the tension roller 17 as the third roller, and the like are driven uniformly so as to drive the motor as the driving unit. Accordingly, the fixing roller 11 is rotated, the pressure roller 12 is rotated, the fixing belt 16 is caused to travel, and the stretching roller 17 is further rotated.

  At this time, before the motor is driven, the pressing force adjustment processing means turns on the plunger 26, separates the pressure pad 20 from the fixing belt 16 against the urging force of the spring 27, and puts it in the retracted position.

  Thereafter, when the surface temperature of the fixing belt 16 reaches a predetermined temperature, the sheet 15 is sent from the image forming unit side to the fixing device. When the sheet 15 reaches a set position immediately before the nip portion, the sheet sensor as a position detecting unit detects the sheet. 15 front ends are detected, and a detection signal is sent to the control unit.

  Then, the pressing force adjustment processing means turns off the plunger 26 and presses the pressure pad 20 against the fixing belt 16 by the urging force of the spring 27 and places it in the operating position.

  As described above, in this embodiment, when the printer is in operation, the pressure pad 20 is placed at the operating position only when the paper 15 is passed and is brought into contact with the fixing belt 16. The sliding time can be shortened. Accordingly, since the wear of the fixing belt 16 and the pressure pad 20 can be suppressed, the durability of the fixing device can be improved, the torque for rotating the fixing roller 11 can be reduced, and the fixing is performed. The reliability of the apparatus over time can be improved.

  Further, since the pressure pad 20 moves along the outer peripheral surface of the fixing roller 11, there is no fear that the pressure pad 20 and the fixing roller 11 interfere with each other.

  In each of the embodiments, the pressure roller 12 is bent by the tension of the fixing belt 16, the second pressing force f2, and the like.

  FIG. 10 is a reference diagram showing the state of the pressure pad.

  As shown in the figure, when the pressure roller 12 is bent, the second pressing force f2 in the longitudinal direction of the pressure roller 12 is not uniform, and is large at both ends of the pressure roller 12 and small at the center. The image quality will be degraded.

  Therefore, a fifth embodiment of the present invention that can suppress the bending of the pressure roller 12 will be described. In addition, about the thing which has the same structure as 2nd-4th embodiment, the description is abbreviate | omitted by giving the same code | symbol, About the effect of invention by having the same structure, the effect of the same embodiment Is used.

  FIG. 11 is a view showing the state of the pressure pad in the fifth embodiment of the present invention, FIG. 12 is a sectional view of both ends of the pressure pad in the fifth embodiment of the present invention, and FIG. It is sectional drawing of the center part of the pressure pad in 5 embodiment.

  In the present embodiment, as shown in the figure, in the pressure pad 20 as a pressing member, the distance from the abutting portion 20a to the back surface portion 20d is shorter at both ends, and is longer at the center in the longitudinal direction, The cross section in the direction perpendicular to the longitudinal direction of the pressure pad 20 is smaller at both ends and larger at the center. The pressure roller 12 is bent by the tension of the fixing belt 16, the second pressing force f2, and the like. However, since the central portion of the pressure pad 20 protrudes toward the pressing roller 12, the second pressing force is increased. The pressure f2 can be made uniform in the longitudinal direction of the pressure pad 20.

  Therefore, it is possible to prevent the occurrence of fixing unevenness, so that the image quality can be improved.

  By the way, in each of the embodiments, the surface temperature of the fixing belt 16 and the pressure roller 12 is such that the sheet 15 passes through the portion where the sheet 15 passes, that is, the sheet 15 passes through the sheet passing portion. Is transmitted and becomes low, and becomes high in a portion where the paper 15 does not pass, that is, in a non-sheet passing portion, and a temperature difference occurs between the sheet passing portion and the non-sheet passing portion. Therefore, when continuous printing is performed, the temperature difference increases and fixing unevenness occurs. In particular, when continuous printing is performed on the narrow paper 15 and then printing is performed on the wide paper 15, the paper passing portion of the narrow paper 15 and the outside of the paper passing portion are The temperature difference from the paper passing portion of the wide paper 15 becomes large, fixing unevenness occurs, and the image quality deteriorates.

  Accordingly, a sixth embodiment of the present invention in which the temperature difference between the sheet passing portion and the non-sheet passing portion in the fixing belt 16 and the pressure roller 12 can be reduced will be described. In addition, about the thing which has the same structure as 1st-5th embodiment, the description is abbreviate | omitted by giving the same code | symbol, About the effect of invention by having the same structure, the effect of the same embodiment Is used.

  FIG. 14 is a cross-sectional view of the fixing device according to the sixth embodiment of the present invention.

  In the figure, reference numeral 35 denotes a pressure pad as a pressing member. The pressure pad 35 has a contact portion 35a that contacts the pressure roller 12 as the second roller via the fixing belt 16 as the fixing member, and the corresponding contact. A contact portion 35 b that is formed on a portion closer to the tension roller 17 than the portion 35 a and contacts the fixing belt 16 is provided. The material of the pressure pad 35 is preferably a good heat conductor having a high thermal conductivity, for example, a metal.

  Next, the operation of the fixing device having the above configuration will be described.

  In this case, when continuous printing is performed on the paper 15 as a recording medium having a narrow width, heat is transmitted to the paper 15 in the paper passing portion of the paper 15 having a narrow width, and the heat is transmitted through the fixing belt 16 and the pressure roller 12. Although the temperature of the paper portion tends to decrease, the heat of the non-paper passing portion of the narrow paper 15 is transmitted to the paper passing portion via the pressure pad 35. As a result, the temperature of the fixing belt 16 and the pressure roller 12 can be made uniform in the longitudinal direction, so that uneven fixing can be prevented and image quality can be improved.

  In the contact portion 35b, the pressure at which the pressure pad 35 and the fixing belt 16 come into contact with each other is lowered, so that the pressure pad 35 and the fixing belt 16 can be prevented from being worn.

  In addition, this invention is not limited to the said embodiment, It can change variously based on the meaning of this invention, and does not exclude them from the scope of the present invention.

1 is a cross-sectional view of a fixing device according to a first embodiment of the present invention. It is sectional drawing of the conventional fixing device. It is sectional drawing of the conventional belt-type fixing device. It is sectional drawing of the fixing device in the 2nd Embodiment of this invention. It is a figure which shows the pressure distribution of the nip part in the 1st Embodiment of this invention. It is a figure which shows the pressure distribution of the nip part in the 2nd Embodiment of this invention. FIG. 9 is a cross-sectional view of a fixing device according to a third embodiment of the present invention. It is sectional drawing which shows the state which put the pressure pad in the operating position in the 4th Embodiment of this invention. It is sectional drawing which shows the state which put the pressure pad in the 4th Embodiment of this invention in the retracted position. It is a reference figure which shows the state of a pressure pad. It is a figure which shows the state of the pressure pad in the 5th Embodiment of this invention. It is sectional drawing of the both ends of the pressure pad in the 5th Embodiment of this invention. It is sectional drawing of the center part of the pressure pad in the 5th Embodiment of this invention. It is sectional drawing of the fixing device in the 6th Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Fixing roller 12 Pressure roller 16 Fixing belt 17 Stretch roller 20, 35 Pressure pad 20a, 35a Contact part 35b Contact part

Claims (9)

(A) a fixing roller;
(B) a stretching roller disposed in parallel with the fixing roller;
(C) an endless fixing belt stretched between the fixing roller and the stretching roller;
(D) a pressure roller disposed outside the fixing belt and pressed against the fixing roller via the fixing belt;
(E) The pressure roller is disposed on the inner side of the fixing belt and is in pressure contact with the pressure roller via the fixing belt at a downstream side of the pressure contact portion where the pressure roller is pressed against the fixing roller in the recording medium conveyance direction. And a pressing member for fixing.   The fixing device according to claim 1, wherein the fixing belt is deflected toward the pressing member at a downstream side of a pressing portion where the pressing roller is pressed against the pressing member in the recording medium conveyance direction.   The fixing device according to claim 1, wherein the pressing member is formed with an R shape at a portion in contact with the pressure roller, and a radius of the R shape is set to 10 mm or less.   4. The pressing force of the pressure contact portion where the pressure roller is brought into pressure contact with the pressing member is made larger than the pressure force of the pressure contact portion where the pressure roller is pressed against the fixing roller. Fixing device.   5. The fixing device according to claim 1, wherein a pressing force of a pressure contact portion where the pressure roller is pressed against the pressing member is changed in accordance with a thickness of the recording medium.   The fixing device according to claim 1, wherein the pressing member is rotatably disposed on the same axis as the shaft of the fixing roller.   7. The pressing member according to claim 1, wherein the pressing member is placed at an operating position that comes into contact with the fixing belt when the recording medium is passed, and is placed at a retracted position that is not in contact with the fixing belt when the recording medium is not passed. The fixing device according to Item.   The fixing device according to claim 1, wherein a cross section in a direction perpendicular to the longitudinal direction of the pressing member is smaller at both end portions and larger at a central portion. 2. The pressing member is made of a good heat conductor and includes a contact portion that contacts the pressure roller via the fixing belt, and a contact portion that is formed on the tension roller side from the corresponding contact portion and contacts the fixing belt. The fixing device according to any one of ˜8.

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