IMAGE FORMING APPARATUS THAT ADJUSTS PRESSURE TO NIP REGION FROM PRESSURE ROLLER, BY SWITCHING BETWEEN NORMAL LEVEL AND LOW LEVEL LOWER THAN NORMAL LEVEL

An image forming apparatus includes an image forming device, a fixing device, and a controller. The fixing device includes a fixing belt, a heater, a pressure roller, a drive device, and a pressure adjustment mechanism that adjusts pressure to the nip region from the pressure roller, by switching between a normal level and a low level. When executing a print job, the controller causes the drive device to rotate the pressure roller, thereby causing the fixing belt to start to rotate, after causing the pressure adjustment mechanism to switch the pressure to the nip region to the low level, causes the pressure adjustment mechanism to switch the pressure to the nip region from the low level to the normal level, when a predetermined time has elapsed after the fixing belt started to rotate, and causes the image forming device to form an image according to the print job.

INCORPORATION BY REFERENCE

This application claims priority to Japanese Patent Application No. 2023-029707 filed on Feb. 28, 2023, the entire contents of which are incorporated by reference herein.

BACKGROUND

The present disclosure relates to a fixing device that fixes, by thermocompression, an image formed on a recording medium such as a recording sheet, and an image forming apparatus including the fixing device.

Existing image forming apparatuses that utilize the electrophotography process, such as a copier or a multifunction peripheral, include a fixing device that fixes the image formed on the recording medium. The fixing device includes a rotatable cylindrical fixing belt, a heater that heats the fixing belt, a heater retention member that holds the heater so as to bring the heater into contact with the inner circumferential surface of the fixing belt, and a pressure roller that holds the fixing belt between itself and the heater, and defines a nip region between itself and the fixing belt, through which the recording medium is transported in a nipped state, the pressure roller being configured to drive the fixing belt to rotate. With such a configuration, the image formed on the recording medium is fixed thereto, by being heated and pressed (thermocompression) in the nip region.

SUMMARY

The disclosure proposes further improvement of the foregoing techniques.

In an aspect, the disclosure provides an image forming apparatus including an image forming device, a fixing device, and a control device. The image forming device forms an image on a recording medium. The fixing device includes a fixing belt, a heater, a pressure roller, a drive device, and a pressure adjustment mechanism. The fixing belt is formed in a cylindrical shape, and a circumferential surface thereof rotates in a circumferential direction. The heater heats the fixing belt from inside thereof. The pressure roller holds the fixing belt between the pressure roller and the heater, thereby defining a nip region, through which the recording medium having the image formed thereon by the image forming device is transported in a nipped state, between the pressure roller and the fixing belt, and causes, by rotating, the fixing belt to rotate. The drive device drives the pressure roller to rotate. The pressure adjustment mechanism adjusts pressure to the nip region from the pressure roller, by switching between a predetermined normal level and a predetermined low level lower than the normal level. The control device includes a processor, and acts as a controller that controls an operation of the heater and the pressure adjustment mechanism, when the processor executes a control program. When executing a print job, the controller causes the drive device to rotate the pressure roller, thereby causing the fixing belt to start to rotate, after causing the pressure adjustment mechanism to switch the pressure to the nip region to the low level, causes the pressure adjustment mechanism to switch the pressure to the nip region from the low level to the normal level, when a predetermined time has elapsed after the fixing belt started to rotate, and causes the image forming device to form an image according to the print job.

In another aspect, the disclosure provides an image forming apparatus including an image forming device, a fixing device, and a control device. The image forming device forms an image on a recording medium. The fixing device includes a fixing belt, a heater, a pressure roller, a drive device, and a pressure adjustment mechanism. The fixing belt is formed in a cylindrical shape, and a circumferential surface thereof rotates in a circumferential direction. The heater heats the fixing belt from inside thereof. The pressure roller holds the fixing belt between the pressure roller and the heater, thereby defining a nip region, through which the recording medium having the image formed thereon by the image forming device is transported in a nipped state, between the pressure roller and the fixing belt, and causes, by rotating, the fixing belt to rotate. The drive device drives the pressure roller to rotate. The pressure adjustment mechanism adjusts pressure to the nip region from the pressure roller, by switching between a predetermined normal level and a predetermined low level lower than the normal level. The control device includes a processor, and acts as a controller that controls an operation of the heater and the pressure adjustment mechanism, when the processor executes a control program. The controller sets a target temperature of the heater to a predetermined high temperature, higher than a normal temperature, after causing the pressure adjustment mechanism to switch the pressure to the nip region from the normal level to the low level.

DETAILED DESCRIPTION

Hereafter, an image forming apparatus according to some embodiments of the disclosure will be described, with reference to the drawings.FIG.1is a functional block diagram schematically showing an essential internal configuration of the image forming apparatus according to a first embodiment of the disclosure. The image forming apparatus1is a multifunction peripheral having a plurality of functions, such as copying, printing, scanning, and facsimile transmission.

The image forming apparatus1includes a control device10, a document feeding device6, a document reading device5, an image forming device12, a fixing device13, a sheet feeding device14, an operation device470, a pressure adjustment mechanism50, a drive device131, and a storage device8.

The document feeding device6is openably connected to the upper face of the document reading device5, for example via a hinge. The document feeding device6serves as a document retention cover, when the document reading device5reads a source document placed on the platen glass. The document feeding device6is configured as an automatic document feeder (ADF) including a document tray, and delivers the source documents placed thereon to the document reading device5.

To perform the document reading operation, the image forming apparatus1operates as follows. The document reading device5optically reads the image on the source document delivered from the document feeding device6to the document reading device5, or placed on the platen glass, and generates image data. The image data generated by the document reading device5is stored, for example, in an image memory.

To perform the image forming operation, the image forming apparatus1operates as follows. The image forming device12forms an image on a recording sheet, serving as a recording medium, and delivered from the paper feeding device14, on the basis of the image data generated through the document reading operation, image data stored in the image memory, or image data received from a computer connected via the network.

The fixing device13heats and presses the recording sheet on which the toner image has been formed by the image forming device12, to thereby fix the toner image on the sheet. The recording sheet that has undergone the fixing process is delivered to an output tray. The sheet feeding device14includes one or more sheet cassettes.

The drive device131includes a drive circuit serving as a drive source for rotating the pressure roller30.

The operation device470includes various hard keys, and receives instructions to execute the functions and operations that the image forming apparatus1is configured to perform, according to inputs made by the user through the hard keys. The operation device470also includes a display device473for displaying, for example, an operation guide for the user. The operation device470receives, through a touch panel provided on the display device473, the user's instruction based on an operation (touch operation) performed by the user on the operation screen displayed on the display device473.

The display device473includes, for example, a liquid crystal display (LCD). The display device473includes the touch panel. When the user touches a button or a key displayed on the screen, the touch panel receives the instruction corresponding to the touched position.

The storage device8is a large-capacity storage device, such as a hard disk drive (HDD) or a solid state drive (SSD), and contains various control programs. The pressure adjustment mechanism50provides the pressing force from the pressure roller30to the fixing belt20, and adjusts the pressure applied to a nip region N.

The control device10includes a processor, a random-access memory (RAM), a read-only memory (ROM), and an exclusive hardware circuit. The processor is, for example, a central processing unit (CPU), an application specific integrated circuit (ASIC), or a micro processing unit (MPU). The control device10includes the controller100.

The control device10acts as the controller100, when the processor operates according to the control program stored in the storage device8. Here, the controller100may be constituted in the form of a hardware circuit, instead of being realized by the operation of the control device10according to the control program. This also applies to other embodiments, unless otherwise specifically noted.

The controller100serves to control the overall operation of the image forming apparatus1. The controller100is connected to the document feeding device6, the document reading device5, the image forming device12, the fixing device13, the sheet feeding device14, the operation device470, the pressure adjustment mechanism50, the drive device131, and the storage device8, and controls the operation of the mentioned components. For example, the controller100controls the operation of the image forming device12and the fixing device13, to execute a print job.

FIG.2is a perspective view showing an example of the fixing device13included in the image forming apparatus1. The fixing device13includes a rotatable cylindrical fixing belt20, and a pressure roller30. The fixing belt20heats the recording medium (recording sheet P), having a toner image formed thereon. The fixing belt20is rotatable about an axial center defined as a first rotation axis A1, and extends in the direction of the first rotation axis A1.

The pressure roller30is rotatable about an axial center defined as a second rotation axis A2parallel to the first rotation axis A1, and extends in the direction of the second rotation axis A2. The pressure roller30defines the nip region N, through which the recording sheet P is transported in a nipped state, between the pressure roller30and the fixing belt20, and drives the fixing belt20to rotate. An arrow D inFIG.2indicates the transport direction of the recording sheet P.

FIG.3is a cross-sectional view schematically showing an example of the fixing device13. The fixing device13includes the fixing belt20, the pressure roller30, a heater21, a heater retention member22, a temperature sensor23, a support member24, a pressing member25, and the pressure adjustment mechanism50. InFIG.3, the detailed structure of the pressure adjustment mechanism50is omitted.

The fixing belt20is driven to rotate in a first rotation direction R1(counterclockwise inFIG.3) about the first rotation axis A1, so as to follow up the rotation of the pressure roller30in a second rotation direction R2(clockwise inFIG.3) about the second rotation axis A2.

The heater21heats the fixing belt20. The heater21is a plane heater extending in the first rotation axis A1, and located inside of the fixing belt20so as to oppose an inner circumferential surface201of the fixing belt20. The heater21may be, for example, a ceramic heater including a ceramic substrate and a resistive heating element.

The heater retention member22retains the heater21. The heater retention member22is formed of a heat-resistant resin material, in a shape having a U-shaped cross-section and extending in the direction of the first rotation axis A1. The heater retention member22includes opposing faces221and222, respectively located on the upstream side and the downstream side in the transport direction D of the recording sheet P, and opposed to the inner circumferential surface201of the fixing belt20.

The temperature sensor23is opposed to the heater21, and detects the temperature of the heater21. The temperature sensor23is inserted in a through hole formed in the heater retention member22, so as to be abutted against the heater21. The temperature sensor23is unfixed to the heater retention member22. For example, a thermistor may be employed as the temperature sensor23.

The support member24is a metal stay having an inverted U-shaped cross-section, and extending in the direction of the first rotation axis A1. The support member24is fixed to the main body housing of the fixing device13. The heater retention member22is attached to the main body housing, so as to move toward and away from the support member24. The support member24supports the posture of the heater retention member22, when the end portion of the support member24on the side of the heater retention member22is in contact with the heater retention member22.

The pressing member25is located between the temperature sensor23and the support member24, and presses the temperature sensor23against the heater21. The pressing member25may be, for example, a coil spring. Here, the mentioned configuration is merely exemplary, and the disclosure is not limited to such configuration.

The pressure adjustment mechanism50provides the pressing force from the pressure roller30to the fixing belt20, and adjusts the pressure G applied to the nip region N. The pressure adjustment mechanism50is configured to switch the pressure G to two levels, namely between a predetermined normal level and a predetermined low level, lower than the normal level.

The pressure G (pressing force from the pressure roller30to the fixing belt20) applied to the nip region N in the low level, by the pressure adjustment mechanism50, is set to a value higher than or equal to the biasing force of the pressure roller25. This is because, when the pressure G to the nip region N is smaller than the biasing force of the pressure roller25, the force for supporting the heater21from below inFIG.3becomes insufficient, and the heater21may pop out from the heater retention member22, which leads to failure in properly defining the nip region N. In this embodiment, it is unlikely that the heater21pops out from the heater retention member22, and therefore there is no need to fix the heater21to the heater retention member22, which keeps the structure of the device from being complicated. In this embodiment, the heater21is retained by the heater retention member22, but not fixed thereto. Accordingly, the heater21can freely move toward the pressure roller30, with respect to the heater retention member22.

Hereunder, the configuration of the pressure adjustment mechanism50will be described.FIG.4is a perspective view showing the pressure adjustment mechanism50and the related parts.FIG.5is a side cross-sectional view showing the fixing device including the pressure adjustment mechanism50. The pressure adjustment mechanism50includes an eccentric cam31, a first pressure cancelling member40, a second pressure cancelling member41, an arm member43, a first elastic member45, and a second elastic member47. The first pressure cancelling member40, the second pressure cancelling member41, and the arm member43are each formed by folding a sheet metal, such as iron, into a predetermined shape.

The eccentric cam31, constituting a part of the pressure adjustment mechanism50, is provided at each of the end portions of the fixing device13in the longitudinal direction. The two eccentric cams31are connected via a cam shaft33, and a drive input gear35is fixed to an end portion of the cam shaft33. The drive input gear35is connected to a drive output gear of a fixing pressure adjustment motor, via a gear train. For example, a stepping motor, the rotation direction and the rotation angle of which can be accurately controlled, may be employed as the fixing pressure adjustment motor.

The first pressure cancelling member40is supported by a frame side plate13A of the fixing device13, so as to pivot about a fulcrum40A. The eccentric cam31is in contact with the upper face of the first pressure cancelling member40, and the upper end of the second pressure cancelling member41is in contact with the face of the first pressure cancelling member40opposite to the face in contact with the eccentric cam31. The second pressure cancelling member41is formed in a reverse L-shape in a side view, and supported by the frame side plate13A of the fixing device13so as to slide in the up-down direction, and includes a slit41A formed in the vertical portion.

The arm member43is supported by the frame side plate13A of the fixing device13, so as to swing about a pivotal shaft43A. A U-shaped bearing43B that rotatably supports the rotary shaft of the pressure roller30is provided, at a generally central portion of the arm member43.

At a distal end portion of the arm member43(left end inFIG.5), a bent portion43C is formed, which is loosely fitted in the slit41A of the second pressure cancelling member41. In addition, a spring abutment43D, protruding downward in a tongue shape, is formed between the bearing43B and the bent portion43C.

The first elastic member45and the second elastic member47are each constituted of a coil spring. The end surface at the upper end of the first elastic member45is abutted against the lower end portion of the second pressure cancelling member41, and the end surface at the lower end of the first elastic member45is abutted against a spring pedestal48of the frame side plate13A. In the end surface at the upper end of the second elastic member47, the spring abutment43D of the arm member43is inserted, and the end surface at the lower end of the second elastic member47is abutted against the spring pedestal48of the frame side plate13A.

FIG.4andFIG.5illustrate the pressure adjustment mechanism50, in the state where the nip pressure is set to the normal level (normal pressure for fixing operation). In the case of the normal level, a minor-diameter portion31A of the eccentric cam31is in contact with the first pressure cancelling member40, and the second pressure cancelling member41is lifted up by a biasing force P1of the first elastic member45, so as to make contact with the first pressure cancelling member40. Accordingly, the bent portion43C of the arm member43, loosely fitted in the slit41A of the second pressure cancelling member41, is also lifted upward.

As result, the arm member43swings clockwise inFIG.5, about the pivotal shaft43A, and therefore the pressure roller30supported by the bearing43B is pressed toward the fixing belt20(direction indicated by a white arrow inFIG.5). At this point, the second elastic member47is in its free length, and a biasing force P2of the second elastic member47is not exerted to the arm member43. Therefore, the nip pressure originating only from the biasing force P1of the first elastic member45is applied to the fixing nip region N.

FIG.6is a partial perspective view showing the end portion of the fixing device13on the opposite side of the portion shown inFIG.4andFIG.5, seen from the outer side. As shown inFIG.6, a cam position sensor51that detects the phase of the eccentric cam31is provided on the outer side of the frame side plate13A. The cam position sensor51is a P1sensor including an optical sensor. In addition, a light blocking member53is fixed to the end portion of the cam shaft33protruding from the frame side plate13A. When the nip pressure is at the normal level, the light blocking member53is located on the upper side of the cam shaft33, and therefore the optical path for the optical sensor of the cam position sensor51is free from an obstacle.

To switch the nip pressure to the low level, the fixing pressure adjustment motor is activated so as to rotate the eccentric cam31by 180° from the state ofFIG.4andFIG.5. Accordingly, a major-diameter portion31B of the eccentric cam31makes contact with the first pressure cancelling member40, thereby causing the first pressure cancelling member40to pivot counterclockwise, about the fulcrum40A, as shown inFIG.7andFIG.8. Owing to such pivotal movement of the first pressure cancelling member40, the second pressure cancelling member41is pressed downward, so as to compress the first elastic member45.

When the second pressure cancelling member41is pressed downward by a predetermined distance, the bent portion43C of the arm member43is separated from the lower end of the slit41A, and therefore the biasing force P1of the first elastic member45is no longer exerted to the arm member43. On the other hand, the biasing force P2of the second elastic member47, abutted against the spring abutment43D, is exerted to the arm member43, and therefore the nip pressure originating only from the biasing force P2of the second elastic member47is applied to the fixing nip region N.

Here, the biasing force P2of the second elastic member47is set to the force corresponding to the pressure of the low level, which is smaller than the biasing force P1of the first elastic member45corresponding to the pressure of the normal level. In addition, even though the biasing forces P1and P2are the same as each other, the biasing force P2of the second elastic member47is exerted at a position closer to the rotary shaft of the pressure roller30, than the first elastic member45, and therefore the force for lifting up the arm member43is reduced, compared with the state shown inFIG.4andFIG.5. Thus, the arm member43swings counterclockwise from the state of the normal level by a predetermined amount, as shown inFIG.8, and the nip pressure is set to the low level, lower than the normal level.

Further, when the nip pressure is switched to the low level, the light blocking member53also revolves by 180° together with the eccentric cam31and the cam shaft33, as shown inFIG.9, thereby being located on the lower side of the cam shaft33, thus blocking the optical path for the optical sensor of the cam position sensor51. In other words, the phase of the eccentric cam31can be detected, on the basis of the photodetection level of the cam position sensor51.

The controller100controls the operation of the heater21. When executing the print job, the controller100causes the drive device131to rotate the pressure roller30so as to cause the fixing belt20to start to rotate, after causing the pressure adjustment mechanism50to switch the pressure to the nip region N to the low level. Further, the controller100causes the pressure adjustment mechanism50to switch the pressure to the nip region N from the low level (hereinafter, low level LS) to the normal level (hereinafter, normal level NP), when a predetermined time has elapsed after the fixing belt20started to rotate, and causes the image forming device12to form an image according to the print job. The predetermined time refers to a time until the drive torque for driving the fixing belt20is lowered and stabilized at a certain level, after the fixing belt20started to rotate. Such time is determined in advance through experiments, and stored, for example, in a non-volatile memory provided in the control device10.

Referring now to a flowchart shown inFIG.10, an exemplary pressure adjustment operation, performed by the image forming apparatus1, will be described hereunder. This operation is performed when the operation device470receives a user's instruction to execute the print job.

The controller100causes the pressure adjustment mechanism50to switch the pressure to the nip region N to the low level LS, according to the instruction to execute the print job, received by the operation device470(S1). The controller100then activates the heater21(S2). Thereafter, the controller100causes the drive device131to rotate the pressure roller30, thereby causing the fixing belt20to rotate so as to follow up the rotation of the pressure roller30(S3).

The controller100then decides whether the predetermined time T has elapsed, after the fixing belt20started to rotate (S4). Upon deciding that the time T has elapsed (YES at S4), the controller100controls the pressure adjustment mechanism50so as to switch the pressure to the nip region N from the low level LS to the normal level NP (S5), and controls the image forming device12and the fixing device13, so as to execute the print job (S6). In this case, it suffices that the controller100causes the pressure adjustment mechanism50to switch from the low level LS to the normal level NP, before the recording sheet P, having the image formed thereon by the image forming device12, comes close to the nip region N of the fixing device13. At this point, the operation is finished.

Now, since the fixing belt defines the nip region in collaboration with the pressure roller as above, a larger drive torque is required to rotate the fixing belt, at the time that the rotation of the fixing belt is started. The increase in drive torque leads to an increase in load imposed on the mechanism for driving the fixing belt to rotate. Accordingly, a technique to suppress the increase in drive torque, by reducing the rotation speed of the fixing belt, has been developed. However, reducing the rotation speed of the fixing belt inevitably leads to degraded productivity.

The drive torque for rotating the fixing belt20becomes largest, at the time that the fixing belt20starts to rotate, and thereafter the drive torque is lowered and stabilized. In this embodiment, therefore, the pressure to the nip region N is switched to the low level LS at the time that the fixing belt20starts to rotate, and then switched from the low level LS to the normal level NP, when the time T elapses, and the recording sheet P having the image formed thereon is about to undergo the fixing operation.

When the pressure to the nip region N is set to the low level LS, the nip region N is narrowed, and therefore the drive torque for rotating the fixing belt20can be reduced, and the maximum value of the drive torque can be suppressed.

Further, the reduction in pressure to the nip region N may lead to degradation in fixing performance, and therefore some measures have to be taken, for example raising the target temperature of the fixing device13, or reducing the printing speed. However, in this embodiment, it is only when the fixing belt20starts to rotate, that the pressure to the nip region N is set to the low level LS, and the pressure to the nip region N is switched to the normal level NP when the fixing operation is performed. Therefore, the fixing operation can be normally performed, without the need to depend on the mentioned measures. Thus, the arrangement according to this embodiment prevents an excessive increase in drive torque at the time that the fixing belt starts to rotate, without compromising the productivity.

FIG.11is a functional block diagram schematically showing an essential internal configuration of an image forming apparatus according to a second embodiment. The image forming apparatus according to the second embodiment is different from the image forming apparatus shown inFIG.1, in that the control device10further includes a counter101and a decider102. The description of the same elements as those of the image forming apparatus shown inFIG.1will not be repeated.

The control device10further acts as the counter101and the decider102, in addition to the controller100, when the processor operates according to the control program stored in the storage device8.

The counter101counts the number of sheets of the recording sheets P on which the image has been formed by the image forming device12(number of sheets printed).

The decider102decides whether a time for switching the pressure to the nip region N from the normal level NP to the low level LS has been reached, on the basis of the state of use of the image forming apparatus1. For example, upon deciding that the number of sheets printed, counted by the counter101, has reached a predetermined number of sheets, the decider102decides that the time for switching has been reached. When the image forming apparatus1is put to use for an extended period of time, the nip region N of the fixing device13may be widened, in which case the drive torque at the time that the fixing belt20starts to rotate may increase. Accordingly, the state of use of the image forming apparatus1can be utilized as an index for deciding whether the time for switching has been reached.

When the decider102decides that the time for switching has been reached, the controller100causes the pressure adjustment mechanism50to switch the pressure to the nip region N from the normal level NP to the low level LS, and sets the target temperature of the heater21to a predetermined high temperature (e.g., 220 degrees Celsius), higher than a normal temperature (e.g., 180 to 210 degrees Celsius). The temperature that enables the fixing operation to be normally performed, despite reducing the nip pressure as above, is detected through experiments carried out before the shipment of the products, and the temperature thus detected is stored in advance in the non-volatile memory provided in the control device10, as the high temperature. With the arrangement according to the second embodiment, the pressure to the nip region N is switched from the normal level NP to the low level LS, about the time that the drive torque is increased, and therefore the excessive increase in drive torque can be effectively suppressed.

When the pressure to the nip region N is lowered, the fixing performance may be degraded, for example owing to decline in thermal conductivity from the fixing belt20to the recording medium. However, since the target temperature of the heater21is raised to the high temperature, higher than the normal temperature, and therefore the degradation in fixing performance due to the reduction of the pressure can be compensated, and the expected fixing performance can be maintained. As result, the arrangement according to the second embodiment also prevents the excessive increase in drive torque, without compromising the productivity.

According to the second embodiment, when the decider102decides that the time for switching has been reached, the controller100switches the pressure to the nip region N from the normal level NP to the low level LS, and also raises the target temperature of the heater21to the high temperature. Alternatively, as another embodiment, the pressure may be switched to the low level LS with the pressure adjustment mechanism50, and the target temperature may be set to the high temperature (e.g., 220 degrees Celsius) in advance, before the shipment of the image forming apparatus1(i.e., before the image forming apparatus1is put to use).

The disclosure may be modified in various manners, without limitation to the foregoing embodiments. Further, the configurations and processings described in the foregoing embodiments with reference toFIG.1toFIG.11are merely exemplary, and in no way intended to limit the disclosure to those configurations and processings.