Fixing device and image forming apparatus incorporating same

A fixing device includes a fixing sleeve, a pair of holders, a heater, a nip formation pad, a pressing member, a pressing mechanism, and a controller. The nip formation pad has a regular crown-shaped surface abutted against an inner surface of the fixing sleeve. The pressing member forms a nip with the nip formation pad in a state of abutting against the fixing sleeve. The controller controls the pressing mechanism in a first mode in which the pressing mechanism presses the pressing member toward the nip formation pad with a first pressing force to equalize a width of the nip and a second mode in which the pressing mechanism presses the pressing member with a second pressing force greater than the first pressing force so that the width of the nip is smaller at both ends than at a center of the nip.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is based on and claims priority pursuant to 35 U.S.C. § 119 to Japanese Patent Application No. 2017-074336, filed on Apr. 4, 2017 in the Japanese Patent Office, the entire disclosure of which is hereby incorporated by reference herein.

BACKGROUND

Technical Field

The present disclosure relates to a fixing device to fix an image on a recording medium and an image forming apparatus incorporating the fixing device.

Background Art

Generally, a fixing device for an image forming apparatus has a fixing member having a heat source and a pressure rotator pressed against the fixing member by a pressing mechanism. When a recording sheet on which a toner image as a developer image is formed passes through a fixing nip where both the fixing member and the pressure rotator are pressed against each other, the toner image is thermally fixed on the recording sheet.

Contemporary fixing devices use a fixing sleeve as a fixing member having a low heat capacity to save energy and shorten the time to first printing.

SUMMARY

This specification describes an improved fixing device and an image forming apparatus including the fixing device.

In one illustrative embodiment, the fixing device includes a fixing sleeve, a pair of holders oppositely disposed in an axial direction of the fixing sleeve to rotatably support both ends of the fixing sleeve in the axial direction, a heater to heat the fixing sleeve, a nip formation pad, a pressing member, a pressing mechanism, and a controller. The nip formation pad is disposed between the pair of holders and has a regular crown-shaped surface abutted against an inner circumferential surface of the fixing sleeve. The pressing member is disposed opposite the fixing sleeve to form a nip with the nip formation pad in a state of abutting against the fixing sleeve, and rotates the fixing sleeve by rotation in the abutting state. The nip has a width in a direction perpendicular to the axial direction of the fixing sleeve. The pressing mechanism presses the pressing member toward the nip formation pad. The controller controls the pressing mechanism in two modes: a first mode, in which the pressing mechanism presses the pressing member toward the nip formation pad with a first pressing force to equalize the width of the nip in a longitudinal direction of the abutted surface, and a second mode, in which the pressing mechanism presses the pressing member toward the nip formation pad with a second pressing force greater than the first pressing force so that the width of the nip is smaller at both ends of the nip in the longitudinal direction than at a center of the nip in the longitudinal direction.

DETAILED DESCRIPTION

Referring now to the drawings, embodiments of the present disclosure are described below. In the drawings illustrating the following embodiments, the same reference codes are allocated to elements (members or components) having the same function or shape and redundant descriptions thereof are omitted below.

Below, a fixing device and an image forming apparatus according to an embodiment of the present disclosure is described below.

Image Forming Apparatus

A description is provided of the construction of an image forming apparatus1000according to an embodiment of the present disclosure.

As illustrated inFIG. 1, in the present embodiment, the image forming apparatus1000is a color laser printer, and includes an image forming section A, a sheet feed section B, a fixing device200, a curl correction device300, a pair of sheet ejection rollers13, and an output tray14. The image forming section A includes four image forming units4Y,4M,4C, and4K, an exposure device9, and a transfer device3. Descriptions are given in further detail below.

The image forming apparatus1000includes four image forming units4Y,4M,4C, and4K disposed in the center of the image forming apparatus1000. Although the image forming units4Y,4M,4C, and4K contain developers (e.g., yellow, magenta, cyan, and black toners) in different colors, that is, yellow, magenta, cyan, and black corresponding to color separation components of a color image, respectively, the image forming units4Y,4M,4C, and4K have an identical structure.

Specifically, each of the image forming units4Y,4M,4C, and4K includes a drum-shaped photoconductor5as a latent image bearer; a charger6to charge a surface of the photoconductor5; a developing device7to supply toner on the surface of the photoconductor5; and a cleaning device8to clean the surface of the photoconductor5.

InFIG. 1, each of the photoconductor5, the charger6, the developing device7, and the cleaning device8included in the black image forming unit4K is supplied with a reference numeral and reference numerals for other image forming units4Y,4M, and4C configured similarly to the image forming unit4K are omitted.

An exposure device9is disposed below the image forming units4Y,4M,4C, and4K and exposes the outer circumferential surfaces of the respective photoconductors5with laser beams. The exposure device9includes a laser light source, a polygon mirror, an f-θ lens, and a plurality of reflection mirrors to irradiate the surface of each photoconductor5with a laser beam based on image data and form an electrostatic latent image on the surface of each photoconductor5.

A transfer device3is disposed above the image forming units4Y,4M,4C, and4K. The transfer device3includes an intermediate transfer belt30as an intermediate transfer member, four primary transfer rollers31as primary transfer members, a secondary transfer backup roller32, a secondary transfer roller36as a secondary transfer member, a cleaning backup roller33, a tension roller34, and a belt cleaner35.

The intermediate transfer belt30is an endless belt stretched taut across the secondary transfer backup roller32, the cleaning backup roller33, and the tension roller34. As a driver drives and rotates the secondary transfer backup roller32counterclockwise inFIG. 1, the secondary transfer backup roller32rotates the intermediate transfer belt30counterclockwise inFIG. 1, that is, in a rotation direction indicated by arrow E inFIG. 1.

The four primary transfer rollers31sandwich the intermediate transfer belt30together with the four photoconductors5, forming four primary transfer nips between the intermediate transfer belt30and the photoconductors5, respectively. In addition, each primary transfer roller31is connected to a power source and a predetermined direct current (DC) voltage or alternating current (AC) voltage is applied to each primary transfer roller31.

The secondary transfer roller36sandwiches the intermediate transfer belt30together with the secondary transfer backup roller32, forming a secondary transfer nip between the secondary transfer roller36and the intermediate transfer belt30. In addition, similarly to the primary transfer rollers31, the secondary transfer roller36is connected to a power source, and a predetermined direct current (DC) voltage or alternating current (AC) voltage is applied to the secondary transfer roller36.

The belt cleaner35includes a cleaning brush and a cleaning blade that contact an outer circumferential surface of the intermediate transfer belt30. A waste toner drain tube extending from the belt cleaner35to an inlet of a waste toner container conveys waste toner collected from the intermediate transfer belt30by the belt cleaner35to the waste toner container.

A bottle holder2disposed in an upper portion of the image forming apparatus1000accommodates four toner bottles2Y,2M,2C, and2K detachably attached to the bottle holder2. The toner bottles2Y,2M,2C, and2K contain fresh yellow, magenta, cyan, and black toners to be supplied to the developing devices7of the image forming units4Y,4M,4C, and4K, respectively. For example, the fresh yellow, magenta, cyan, and black toners are supplied from the toner bottles2Y,2M,2C, and2K to the developing devices7through toner supply tubes interposed between the toner bottles2Y,2M,2C, and2K and the developing devices7, respectively.

The sheet feed section B is disposed in the bottom of the image forming apparatus1000. The sheet feed section B includes a sheet tray10in which a recording medium P as a sheet material is contained, and a sheet feed roller11to feed the recording medium P from the sheet tray10.

The recording medium P may be thick paper, postcards, envelopes, plain paper, thin paper, coated paper, art paper, tracing paper, overhead projector (OHP) transparencies, and the like. The image forming apparatus1000may include a bypass feeder. According to the present embodiment, a sheet P having a basis weight of 160 grams/m2or more is defined as thick paper.

Further, a conveyance path R is disposed inside a body100of the image forming apparatus1000. Through the conveyance path R, the recording medium P is conveyed from the sheet tray10to an outside of the body100via the secondary transfer nip. A registration roller pair12serving as a timing roller to convey the recording medium P to the secondary transfer nip at an appropriate timing for conveyance is disposed upstream from the secondary transfer roller36in the recording medium conveyance direction in the conveyance path R.

The fixing device200presses and heats the recording medium P on which an unfixed image is borne and thereby fixes the toner image onto the recording medium P. The fixing device200is disposed downstream from the position of the secondary transfer roller36in the recording medium conveyance direction. Further, a pair of sheet ejection rollers13to eject the recording medium P outside the body100of the image forming apparatus1000is disposed downstream from the fixing device200in the recording medium conveyance direction of the conveyance path R. In addition, the output tray14to stock the recording medium P ejected outside the image forming apparatus1000is disposed on an upper surface of the body100of the image forming apparatus1000.

Basic Operation of Image Forming Apparatus

Next, basic operation of the image forming apparatus1000according to an embodiment of the present disclosure is described.

As a print job starts, a driver drives and rotates the photoconductors5of the image forming units4Y,4M,4C, and4K, respectively, clockwise inFIG. 1. The chargers6uniformly charge the outer circumferential surface of the respective photoconductors5at a predetermined polarity. The exposure device9emits laser beams onto the charged outer circumferential surfaces of the respective photoconductors5, respectively, thus forming electrostatic latent images on the photoconductors5.

The image data used to expose the respective photoconductors5is monochrome image data produced by decomposing a desired full color image into yellow, magenta, cyan, and black image data. The developing devices7supply yellow, magenta, cyan, and black toners to the electrostatic latent images formed on the photoconductors5, visualizing the electrostatic latent images as yellow, magenta, cyan, and black toner visible images, respectively.

Simultaneously, as the print job starts, the secondary transfer backup roller32is driven and rotated counterclockwise inFIG. 1, and rotates the intermediate transfer belt30in the rotation direction illustrated by arrow E inFIG. 1. The power supply applies a constant voltage or a constant current control voltage having a polarity opposite a polarity of the charged toner to the primary transfer rollers31, creating transfer electric fields at the respective primary transfer nips formed between the photoconductors5and the primary transfer rollers31.

The transfer electrical fields generated in the primary transfer nips transfer and superimpose the toner images from the respective photoconductors5onto the intermediate transfer belt30to form a full color image on the outer circumferential surface of the intermediate transfer belt30.

After the primary transfer of the yellow, magenta, cyan, and black toner images from the photoconductors5onto the intermediate transfer belt30, the cleaning device8removes residual toner that has failed to be transferred onto the intermediate transfer belt30and has remained on the photoconductors5. Thereafter, dischargers discharge the outer circumferential surfaces of the respective photoconductors5, returning the outer circumferential surfaces of the respective photoconductors5to their initial surface potential.

In the meantime, the sheet feed roller11disposed in the lower portion of the image forming apparatus1000is driven and rotated to feed the recording medium P from the sheet tray10toward the registration roller pair12through the conveyance path R. The registration roller pair12temporarily halts the recording medium P conveyed through the conveyance path R.

Thereafter, the registration roller pair12resumes rotation at a predetermined time to convey the recording medium P to the secondary transfer nip at a time when the image formed on the intermediate transfer belt30reaches the secondary transfer nip. The secondary transfer roller36is supplied with a transfer voltage having a polarity opposite a polarity of the charged yellow, magenta, cyan, and black toners constructing the image formed on the intermediate transfer belt30, thus creating a transfer electric field at the secondary transfer nip.

The transfer electric field secondarily transfers the superimposed yellow, magenta, cyan, and black toner images constructing the image formed on the intermediate transfer belt30onto the recording medium P. After the secondary transfer of the full color toner image from the intermediate transfer belt30onto the recording medium P, the belt cleaner35removes residual toner that has failed to be transferred onto the recording medium P and has remained on the intermediate transfer belt30. The removed toner is conveyed to and collected in the waste toner container.

Thereafter, the recording medium P is conveyed to the fixing device200, and the fixing device200fixes the toner image on the recording medium P. The recording medium P conveyed from the fixing device200is ejected to the output tray14outside the body100of the image forming apparatus1000, after passing through the curl correction device300.

The above describes the image forming operation of the image forming apparatus1000to form the full color image on the recording medium P. Alternatively, the image forming apparatus1000may form a monochrome toner image by using any one of the four image forming units4Y,4M,4C, and4K or may form a bicolor toner image or a tricolor toner image by using two or three of the image forming units4Y,4M,4C, and4K.

Fixing Device

Next, a description is given of the fixing device200according to the present embodiment of the present disclosure.

As illustrated inFIG. 2, the fixing device200includes a fixing sleeve201serving as a rotatable and endless fixing member and a pressure roller203serving as a pressing member that is rotatable, disposed opposite the fixing sleeve201, and contacting an outer circumferential surface of the fixing sleeve201.

Inside the fixing sleeve201, a first halogen heater202A and a second halogen heater202B (collectively referred to as halogen heaters202) serve as a fixing heat source. The halogen heaters202directly heat a region other than the nip portion of the fixing sleeve201, that is, a region illustrated by an angular range a and an angular range β inFIG. 1, with radiant heat from the inner circumferential side.

Inside the fixing sleeve201, a nip formation pad206is disposed closer to the pressure roller203than the halogen heater202. On the front side of the nip formation pad206, a heat transfer member216is disposed as described later inFIG. 3. A nip portion N is formed between the nip formation pad206and the pressure roller203.

Fixing Sleeve

The fixing sleeve201is an endless belt or film made of metal such as nickel and SUS stainless steel or resin such as polyimide. A pair of holders208oppositely disposed in the axial direction holds both ends of the fixing sleeve201rotatably. A surface layer of the fixing sleeve201has a release layer made of tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), polytetrafluoroethylene (PTFE), or the like to facilitate separation of toner and avoid toner adherence to the fixing sleeve201.

An elastic layer made of silicone rubber or the like may be sandwiched between the base layer and the release layer such as a PFA layer or a PTFE layer. If the fixing sleeve201does not incorporate the elastic layer, the fixing sleeve201has a decreased thermal capacity that improves fixing property of being heated quickly to a desired fixing temperature at which the toner image is fixed on the recording medium. However, as the pressure roller203and the fixing sleeve201sandwich and press the unfixed toner image on the recording medium passing through the fixing nip N, slight surface asperities of the fixing sleeve201may be transferred onto the toner image on the recording medium, resulting in variation in gloss of the solid toner image that may appear as an orange peel image on the recording medium. To address this circumstance, the elastic layer made of silicone rubber has a thickness equal to or larger than about 100 micrometers. As the elastic layer deforms, the elastic layer absorbs slight surface asperities of the fixing sleeve201, preventing formation of the faulty orange peel image.

Pressure Roller

In the present embodiment, the pressure roller203is constructed of a core bar205, an elastic rubber layer204coating the core bar205, and a surface release layer coating the elastic rubber layer204and made of PFA or PTFE to facilitate separation of the recording medium from the pressure roller203. As a driving force generated by a driver (e.g., a motor) disposed inside the body100of the image forming apparatus1000that is depicted as a printer inFIG. 1is transmitted to the pressure roller203through a gear train, the pressure roller203rotates.

A pressing mechanism400described later presses the pressure roller203against the nip formation pad206via the fixing sleeve201, that is, the pressure roller203is in an abutting state with the fixing sleeve201. As the pressing mechanism400presses and deforms the elastic rubber layer204of the pressure roller203, the pressure roller203produces the fixing nip N having a predetermined length in the recording medium conveyance direction S.

Pressing Mechanism

A pressing mechanism400is configured to presses the pressure roller203against the nip formation pad206via the fixing sleeve201. The pressing mechanism400includes a pressing arm404that presses a core bar205of the pressure roller203toward the nip formation pad206and a pressure spring405coupled to the pressing arm404.

One end of the pressing arm404on the left side inFIG. 2is pivotally supported by a fixed support shaft, and the lower end of the pressure spring405is attached to the other end of the pressing arm404on the right side inFIG. 2. The pressure roller203receives a strong urging force by the pressure spring405via the pressing arm404and presses against the fixing sleeve201.

The pressing mechanism400according to the present embodiment includes a stepping motor416and a movable bracket417that changes the urging force of the pressure spring405. The upper end of the pressure spring405is coupled to the lower end of the movable bracket417.

A threaded rotary shaft418rotated by a stepping motor416is vertically threaded into the upper end portion of the movable bracket417. Forward and reverse rotation, as an arrow Q inFIG. 2, of the threaded rotary shaft418by the stepping motor416enables the movable bracket417to move vertically as arrow T inFIG. 2.

Nip Formation Pad, Heat Transfer Member, and Stay

As illustrated inFIG. 3, the nip formation pad206is a rectangular parallelepiped elongated in the axial direction and formed by a heat resistant resin or the like, and the heat transfer member216is disposed on the front side of the rectangular parallelepiped. On the back side of the nip formation pad206, a stay207having sufficient flexural rigidity to withstand the pressing force of the pressure roller203is disposed.

These three members have lengths extending in the width direction or the axial direction (hereinafter referred to as “longitudinal direction”) of the fixing sleeve201. Both ends of the stay207are supported by a pair of holders208that hold both ends of the fixing sleeve201rotatable.

Cut-out recesses206aand206bare formed at both end portions of the nip formation pad206, and lateral end heaters226aand226bto compensate heat quantity of the halogen heaters202are disposed in the recesses206aand206b. Between the lateral end heaters226aand226b, a flat opposing reference surface206copposed to the pressure roller203is set, and the opposing reference surface206cand the lateral end heaters226aand226bare covered with the heat transfer member216.

The heat transfer member216to equalize a temperature of the nip portion N in the axial direction equalizes a temperature of the fixing sleeve201in contact with the opposing reference surface206cin the axial direction and prevents local temperature rise caused by the heat of the lateral end heaters226aand226b. An opposing face216aof the front face of the heat transfer member216becomes a nip formation face that directly contacts the inner surface of the fixing sleeve201, but, since the heat transfer member216is made of a thin sheet metal, from the aspect of mechanical strength, the opposing reference surface206cof the nip formation pad206becomes the substantial nip formation face, and a pressing face that presses the inner circumferential surface of the fixing sleeve201.

The above described stay207serving as a support that supports the nip formation pad206to form the fixing nip N is disposed inside the loop formed by the fixing sleeve201. As the nip formation pad206receives pressure from the pressure roller203, the stay207supports the nip formation pad206to prevent bending of the nip formation pad206and produce an even nip width in an axial direction.

The stay207has a shape having a projection projected from the opposite face to the fixing nip N side. The projection separates a first halogen heater202A and a second halogen heater202B as fixing heat sources from each other. These two halogen heaters202directly heat the inner surface of the fixing sleeve201with radiant heat. Disposing each of halogen heaters202inside the fixing sleeve201makes it easy to make the compact fixing device200including the rotatable endless fixing sleeve201.

The stay207is mounted on and held by flanges serving as a holder208at both lateral ends of the stay207in a longitudinal direction thereof parallel to the axial direction of the fixing sleeve201, respectively, thus being positioned inside the fixing device200. The reflector209interposed between the two halogen heaters202and the stay207prevents the stay207from being heated by each of the halogen heaters202with radiant heat and the like and thereby reducing waste of energy.

Operation of Fixing Device

As the pressure roller203rotates in the rotation direction R1, the fixing sleeve201rotates in the rotation direction R2in accordance with rotation of the pressure roller203by friction therebetween. As the driver drives and rotates the pressure roller203, the driving force of the driver is transmitted from the pressure roller203to the fixing sleeve201at the fixing nip N, thus rotating the fixing sleeve201by friction between the pressure roller203and the fixing sleeve201in the present embodiment ofFIG. 2. At the fixing nip N, the fixing sleeve201is sandwiched between the pressure roller203and the nip formation pad206and rotates; at a circumferential span of the fixing sleeve201other than the fixing nip N, the fixing sleeve201is guided by a flange of a pair of holders208disposed opposite to each other in the axial direction thereof and rotates. The fixing device200is a quick start up (QSU) type fixing device configured as described above, which shortens the warm-up time.

Regular Crown Shape of the Nip Formation Pad

Specifically, as illustrated inFIG. 4, the nip formation pad206is formed into a regular crown shape so that the thickness at the center in the longitudinal direction is maximized.

For example, in the nip formation pad206according to a comparative example, the opposing reference surface206copposed to the pressure roller203is constituted as a flat surface in the longitudinal direction. Pressing force applied to the both ends of the pressure roller203from the pressing mechanism400deforms both end portions of the pressure roller203larger than a center portion of the pressure roller203on the opposing reference surface206c. This pressing force also pushes the nip formation pad206and bends the center portion of the nip formation pad206in the longitudinal direction into a concave shape. Therefore, the increase in the pressing force of the pressure roller203increases the lateral end nip width, but always tends to leave the center nip width narrower as illustrated by the broken line inFIG. 5.

In the embodiment of the present disclosure, the opposing reference surface206cof the nip formation pad206is formed into the regular crown shape as illustrated inFIG. 4. In other words, the nip formation pad206has a shape that becomes thicker continuously symmetrically and smoothly from both ends to the center. The thickness of the nip formation pad206is greatest at the center portion in the longitudinal direction thereof. The pressing mechanism400described above is configured to press both ends of the pressure roller203toward the nip formation pad206, as indicated by the arrows inFIG. 4, and the controller413adjusts the magnitude of the pressing force.

When an ordinary sheet is used, the controller413employs a first mode, that is, it applies small pressing force as a pressing force P1to the pressure roller203, and sets the nip width evenly in the longitudinal direction as illustrated in an one-dot chain line inFIG. 5. When a large-size sheet is used after small-size sheets continuously used, the controller413employs a second mode, that is, the controller413changes the pressing force larger (P1→P2) and applies a large pressing force as a second pressing force P2to the pressure roller203. As a result, the controller413sets the nip width at the center portion larger and the nip width at the end portions smaller than the nip width in the first mode as illustrated in a solid line inFIG. 5. In the second mode, heat transfer from the nip portion N to the both end portions of the large-size sheet decreases, therefore, the fixing failure (hot offset) caused by the temperature rise at lateral ends is prevented.

Here, the small-size sheet means a recording medium having a lateral width D1narrower than the longitudinal length of the halogen heater202as the heat source. The large-size sheet means a recording medium having a lateral width D2larger than the lateral width D1of the small-size sheet.

FIGS. 6A and 6Billustrate a configuration in which the rigidity of the nip formation pad206is made different between the longitudinal center portion and both end portions. The pressing force of the pressure roller203may dent the center portion of the nip formation pad206having only the regular crown shape illustrated inFIG. 4. Therefore, as illustrated inFIGS. 6A and 6B, the center portion206dof the nip formation pad206is made of a material having large rigidity, and both end portions206eof the nip formation pad206are made of a material having lower rigidity than the material of the center portion206d. In this configuration, as illustrated inFIG. 6B, pressure from the pressure roller203to the nip formation pad206that presses the nip formation pad206in a direction of arrows inFIG. 6Ballows bending the end portions206ebut does not bend the center portion206das much, and as a result, decreases the lateral end nip width.

As a specific material of the nip formation pad206, the both end portions206emay be made of polypropylene (PP) resin having low rigidity, and the center portion206dmay be made of liquid crystal polymer (LCP) resin having high rigidity.

However, assembling different rigid members to one nip formation pad206is not easy. Therefore, as illustrated inFIGS. 7A and 7B, a member of the center portion206dand members of the both end portions206eand206f, each of which has different rigidity, may be arranged in the longitudinal direction of the nip formation pad206, and be integrally assembled to the heat transfer member216. Such a configuration as illustrated inFIGS. 7A and 7Bcan reduce the lateral end nip width and waiting time for large-size sheet printing after small-size continuous sheet printing. InFIG. 7B, the nip formation pad206also includes both end portions206g, and the stay207includes a connecting plate207B.

InFIG. 8, the stay207that supports the nip formation pad206has the regular crown shape in which the center portion in the longitudinal direction protrudes toward the pressure roller203similarly to the nip formation pad206. This configuration enables the stay207to support the center portion of the nip formation pad206, increase rigid strength of the center portion of the nip formation pad206, and keep distribution of the nip width as illustrated by the solid line inFIG. 5securely.

FIG. 9illustrates an example of pressing control in the second mode by the controller413. InFIG. 9, the controller413continuously increases the magnitude of the pressing force in accordance with the number of continuous printing small-size sheets transmitted from the sheet information input mechanism412. That is, as the number of continuous printing small-size sheets increases from the small number (N1) to the large number (N2) inFIG. 9, the temperature at lateral ends rises. Therefore, in order to prevent the influence of the temperature rise at lateral ends, the controller413gradually increases the pressing force of the pressure roller203, that is, increases a pressing force of the pressure roller203in proportion to an increase in the number of the small-size sheets, and makes the end nip width relatively smaller than the center portion nip width. That is, a ratio NRof the lateral end nip width NL1to the center nip width NL2(NR=NL2/NL1) decreases from the ratio NR1at the small number of printing N1to the ratio NR2at the large number of printing N2(NR1>NR2). The controller413controls the ratio NRin the second mode smaller than the one in the first mode.

This control effectively decreases the temperature rise at lateral ends and avoids increase of the waiting time and the fixing failure of the large-size sheet after small-size sheet printing. The increase of the center nip width leads to increase of heat given to a center portion of the large-size sheet, and this enables setting lower fixing temperature. Setting the lower fixing temperature saves energy. Although this effect is based on an inherent countermeasure, in the case of a medicine bag machine or the like, this second mode is important and useful.

Instead of counting the number of continuous printing small-size sheets, the controller413may control the magnitude of the pressing force based on a temperature detected by a temperature sensor disposed at the lateral end portion of the fixing sleeve201at which the temperature rise at the lateral ends occurs. However, since additional cost is required for installing the temperature sensor, it is preferable to examine a relation between the number of continuous printing small-size sheets and the temperature at lateral end beforehand by experiment or the like, determine a predetermined number of printing small-size sheets in which the pressing force is increased in the second mode based on the experiment, and control the pressing force according to the number of continuous printing small-size sheets. This is advantageous in reducing the cost of the fixing device. Since a deviation of a surface pressure/a nip width affects sheet conveyance, it is desirable not to change the relation between the surface pressure/the nip width between the center and the lateral end more than necessary.

The present disclosure is not limited to the details of the embodiments described above, and various modifications and improvements are possible.