Fixing apparatus

In an embodiment of the present invention, a pressure application mechanism portion is provided with a pressure-varying link structure arranged so as to be movable in lateral directions along an upper end portion of a device frame, a drive shaft having an eccentric cam for moving the pressure-varying link structure in the lateral directions, a shaft holding structure arranged so as to be rotatable coupled to the pressure-varying link structure, and a coil spring for applying a biasing force to the shaft holding structure in one direction along the upper end portion of the device frame, and a rotational movement is carried out to make a pressure roller approach and move away from a heat roller so as to achieve a relatively low pressure during a color processing mode and a relatively high pressure during a monochrome processing mode.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2005-322436 filed in Japan on Nov. 7, 2005, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to fixing apparatuses of image forming apparatuses in which a color processing mode or a monochrome processing mode in printing process is selectable.

2. Related Art

In recent years, the print processing speeds of image forming apparatuses have been becoming faster. Furthermore, in addition to conventional monochrome printing image forming apparatuses, development of color printing image forming apparatuses has been advancing. As an example of color printing image forming apparatuses there are image forming apparatuses that use an intermediate transfer system in which a color image is formed on a sheet of paper by using a plurality of electrostatic latent image carriers with color separated image information of a plurality of colors (for example, four colors of K, C, M, and Y) and overlaying and transferring these onto an intermediate transfer belt that rotates and contacts the electrostatic latent image carriers with a predetermined pressure, after which transfer of the images as a combined whole is carried out onto a sheet (sheet of paper) that is transported from a paper feed portion (paper feed cassette for example).

In the above-described image forming apparatus, there is a gap in the number of print processing sheets between color processing mode and monochrome processing mode, and it is common for monochrome processing mode to have processing speeds generally 2 to 3 times faster compared to color processing mode.

In the fixing apparatus installed in this image forming apparatus, the power supplied to the heaters of the heat roller and the pressure roller is determined based on the total power consumption determined for the image forming apparatus.

However, in an image forming apparatus that has processing modes in which the number of sheets being passed varies, it is difficult to achieve appropriate fixing performance corresponding to each processing mode when the power consumption of the fixing apparatus is constant. That is, although it also depends on power consumption, when the power consumption is set so as to obtain an appropriate fixing performance in a slow processing mode, there is the possibility that fixing defects will be produced in the fast processing mode, and conversely, when the power consumption is set so as to obtain an appropriate fixing performance in the fast processing mode, there is the possibility that excessive fixing will occur in the slow processing mode. In attempting to deal with such fixing defects and excessive fixing without varying the power consumption, it is conceivable to change the pressing force of the heat roller and the pressure roller according to the processing mode.

In this case, a fixing apparatus described in JP H8-6426A is proposed as a mechanism for adjusting the pressing force of the heat roller and the pressure roller. With this fixing apparatus, a journal bearing is mounted on a pressure lever and the pressure roller can slide vertically inside a slit provided in a lower frame. Furthermore, the pressure lever can swing being pivotably mounted on a shaft and by applying a pressure spring on the end thereof, the pressure roller is caused to press against the fixing roller. Furthermore, two shaft bearing holes are provided for the pressure lever and by varying the bearing hole into which the shaft is inserted, a distance between the shaft (fulcrum of the pressure lever) and the point of application of the pressure spring is changed, which enables the pressing force to be varied.

With the fixing apparatus described in JP H8-6426 A, although the pressing force of the heat roller and the pressure roller can be adjusted in such ways as varying the shaft bearing holes into which the shaft is inserted and by applying an actual pressure spring, these adjustments involve merely adjusting the pressing force of the heat roller and the pressure roller to an always-constant pressure, and since the adjustments are manual adjustments, this does not involve adjusting the pressing force of the heat roller and the pressure roller according to the processing mode.

SUMMARY OF THE INVENTION

An object of the present invention is to provide in an image forming apparatus in which there is a gap in the number of print processing sheets between a color processing mode and a monochrome processing mode, and in which the monochrome processing mode has a processing speed generally 2 to 3 times faster compared to the color processing mode, a fixing apparatus capable of securing sufficient fixing performance by flexibly responding to each processing mode.

A fixing apparatus according to the present invention is a fixing apparatus of an image forming apparatus in which a color processing mode or a monochrome processing mode can be selected in print processing, the fixing apparatus comprising: a heat roller and a pressure roller that, by sandwiching therebetween a sheet of recording paper on which a development image has been transferred and rotating, transport the sheet of recording paper and carry out application of heat and application of pressure on the sheet of recording paper, wherein a pressing force of a nip area, which is a contact area between the heat roller and the pressure roller, can be varied in response to the processing mode. Specifically, the pressing force of the nip area is set to a first predetermined pressure during the color processing mode and is set to a second predetermined pressure higher than the first predetermined pressure during the monochrome processing mode.

That is, during the color processing mode in which sheets are passed with a slow speed, fixing is carried out with a narrow nip width by setting the pressing force of the nip area to the first predetermined pressure (relatively low pressure), and during the monochrome processing mode in which sheets are passed with a fast speed, fixing is carried out with a wide nip width by setting the pressing force of the nip area to the second predetermined pressure (relatively high pressure). In this way, the problem of an insufficient amount of heat is addressed by lengthening the time in which the sheets of recording paper are in contact with the nip area. Thus, stable fixing processing can be carried out in each processing mode.

Here, the pressing force of the nip area may be set to the second predetermined pressure corresponding to the monochrome processing mode at a normal time, and may be changed from the second predetermined pressure to the first predetermined pressure at a time of the color processing mode. When considering the usage conditions of image forming apparatuses, in an ordinary household monochrome printing is used much more than color printing for example. And in the case of usage in an office, monochrome printing is used more than color printing. In view of such usage conditions, it is effective to set the usual standby status to the monochrome processing mode. And when the usual standby status is set to the monochrome processing mode, the contact pressure of both rollers is relatively high and the nip width is also relatively wide, and therefore the thermal conductivity from the heat roller to the pressure roller is improved. Consequently, even when a heater is mounted inside the pressure roller itself, the heating time of that heater can be shortened by a corresponding amount, and therefore power consumption as a whole can be suppressed.

Furthermore, a fixing apparatus according to the present invention may be further provided with a pressure application mechanism portion that causes a movement of pressing the pressure roller against the heat roller so as to change from the second predetermined pressure to the first predetermined pressure at a time of the color processing mode. The pressure application mechanism portion may be configured to cause a movement of pressing the pressure roller such that a contact region of the nip area in a direction perpendicular to a recording paper transport direction is substantially uniformly increased by increasing the pressing force.

Furthermore, the pressure application mechanism portion may be configured to cause a movement of pressing the pressure roller such that a contact region of the nip area in a direction perpendicular to a recording paper transport direction is increased on a downstream side of the recording paper transport direction by increasing the pressing force. When configured in this manner, the contact point (the insertion point of the nip area) of the heat roller and the pressure roller on the side where the leading edge portion of a sheet of recording paper enters the nip area does not move in a direction perpendicular to the recording paper transport direction even though the pressing force changes, and therefore the distance from where the leading edge portion of the sheet of recording paper makes contact with the heat roller until where it is inserted to the nip area is kept constant. That is, insertion of the sheet of recording paper to the nip area can be carried out stably.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<Overall Description of Image Forming Apparatus>

First, an overall structure of an image forming apparatus to which the present invention is applied is described.

FIG. 1is a lateral view of an image forming apparatus in which one embodiment of a fixing apparatus according to the present invention has been applied.

An image forming apparatus10is a color laser printer that records a color image on a sheet of recording paper and is provided with items such as an exposing apparatus1, image forming stations Pa, Pb, Pc, and Pd, an intermediate transfer belt apparatus2, a fixing apparatus3, a paper transport apparatus4, and a paper feed apparatus5.

The image forming stations Pa, Pb, Pc, and Pd form toner images of black (K), cyan (C), magenta (M), and yellow (Y) respectively, and the toner image of each color is transferred to an intermediate transfer belt11of the intermediate transfer belt apparatus2. The image forming stations Pa, Pb, Pc, and Pd are provided with items including development apparatuses21ato21d, photosensitive drums23ato23d, charging units24ato24d, and cleaning units25ato25d.

The photosensitive drums23ato23dpress on primary transfer rollers26ato26drespectively with interposition of the intermediate transfer belt11and rotate with the intermediate transfer belt11at a peripheral speed equivalent to the intermediate transfer belt11, which rotationally moves in the direction of arrow B. Furthermore, the primary transfer rollers26ato26dalso rotate following the intermediate transfer belt11at a peripheral speed equivalent to the intermediate transfer belt11, which rotationally moves in the direction of arrow B.

The charging units24ato24dare roller-type or brush-type units that contact the photosensitive drums23ato23d, or charger-type devices, and uniformly charge the surfaces of the photosensitive drums23ato23d.

The exposing apparatus1is provided with a laser light source1athat irradiates laser light toward the respective photosensitive drums23ato23dand a plurality of mirrors1bthat guide the laser light onto the respective photosensitive drums23ato23d. The laser lights are irradiated onto the surfaces of the respective photosensitive drums23ato23dwhile being modulated in accordance with the image data, such that respective electrostatic latent images are formed on the surfaces of the respective photosensitive drums23ato23d.

It should be noted that a writing head in which light-emitting elements such as ELs and LEDs are arranged in an array may be used as the exposing apparatus1.

The development apparatuses21ato21dstore toner of their respective colors and form toner images of these colors on the surfaces of the photosensitive drums23ato23dby causing toner of these colors to adhere to the electrostatic latent images on the surfaces of the photosensitive drums23ato23d. These toner images are transferred from the photosensitive drums23ato23dand superimposed on the intermediate transfer belt11.

The intermediate transfer belt apparatus2is provided with items such as the intermediate transfer belt11, the primary transfer rollers26ato26d, a drive support roller31, an idler support roller32, and a secondary transfer roller33. The intermediate transfer belt11is rotatably supported by being wound around the drive support roller31and the idler support roller32, and the primary transfer rollers26ato26dand the secondary transfer roller33are pressed against the intermediate transfer belt11.

The intermediate transfer belt11is made of a synthetic resin film of a thickness in the range of 100 μm to 150 μm for example. The secondary transfer roller33is supported so as to be movable laterally, and when it is moved rightward it sandwiches the intermediate transfer belt11between the secondary transfer roller33and the drive support roller31and forms a nip area. The drive support roller31fulfills a role of being a backup roller of the secondary transfer roller33and is driven to rotate in the downstream of the respective nip areas between the primary transfer rollers26ato26dand the photosensitive drums23ato23dso that the intermediate transfer belt11is pulled and made to rotationally move in a direction of arrow B. In this way, the nip areas are maintained stably.

It should be noted that, of the primary transfer rollers26ato26dand the photosensitive drums23ato23d, it is preferable for one of these to be formed of a hard material and the other to be formed of an elastic material in order to more stably form the respective nip areas between the primary transfer rollers26ato26dand the photosensitive drums23ato23d.

Each of the primary transfer rollers26ato26dis made of a metal shaft of a diameter in the range of 8 mm to 10 mm for example, the circumference of which is covered by a conductive elastic material (such as EPDM and urethane foam). With the intermediate transfer belt11sandwiched in the nip areas between the primary transfer rollers26ato26dand the photosensitive drums23ato23d, a bias voltage having a polarity opposite to the charged polarity of the toners is applied to the primary transfer rollers26ato26dsuch that the respective electrical fields are effected with interposition of the intermediate transfer belt11to the toner on the surfaces of the photosensitive drums23ato23d, such that the toner on the surfaces of the photosensitive drums23ato23dis attracted and transferred to the intermediate transfer belt11. Thus, the toner images of respective colors are transferred to the intermediate transfer belt11and superimposed.

It should be noted that brushes or the like may be used instead of rollers as the primary transfer rollers26ato26d.

A cleaning apparatus34includes, for example, a cleaning blade that slides in contact with the surface of the intermediate transfer belt11, and removes toner remaining on the surface of the intermediate transfer belt11to prevent such defects as fogging of the next image to be printed.

In this way, the toner images of respective colors that are transferred and superimposed onto the intermediate transfer belt11are transported to the nip area between the drive support roller31and the secondary transfer roller33in accordance with rotational movement of the intermediate transfer belt11. Then, a leading edge of the toner image of the colors on the intermediate transfer belt11and a leading edge of the sheet of recording paper transported by a registration roller8are aligned, and the toner image of the colors and the sheet of recording paper are superimposed so that the toner image of the colors is transferred to the sheet of recording paper.

After transfer, the sheet of recording paper is transported to the fixing apparatus3and is here sandwiched between a heat roller3aand a pressure roller3b. Thus, the toner of the colors on the sheet of recording paper is thermally melted and mixed so that the toner image of the colors is fixed to the sheet of recording paper as a color image.

After fixing, the sheet of recording paper is transported to a discharge tray35by the paper transport apparatus4and discharged here facedown.

On the other hand, in the image forming apparatus10, the sheets of recording paper are stacked and stored in a paper feed cassette6. In the paper transport apparatus4, sheets of recording paper in the paper feed cassette6are drawn out sheet by sheet by a pickup roller7-1and the sheets of recording paper are transported to the registration roller8by transport rollers4-1.

Also, sheets of recording paper are loaded in a manual paper feed tray9. In the paper feed apparatus5, sheets of recording paper in the manual paper feed tray9are drawn out by a pickup roller7-2and the sheets of recording paper are transported to the registration roller8of the paper transport apparatus4by transport rollers4-7and4-8.

In the paper transport apparatus4, the sheet of recording paper is temporarily stopped by the registration roller8, the leading edge of the sheet of recording paper is aligned, and the sheet of recording paper is transported to the secondary transfer roller33by the registration roller8with a timing such that the leading edge of the sheet of recording paper overlaps the leading edge of the toner image formed on the intermediate transfer belt11of the intermediate transfer belt apparatus2.

It should be noted that it is also possible to use only the image forming station Pa to form a monochrome image and transfer the monochrome image to the intermediate transfer belt11of the intermediate transfer belt apparatus2. As with the color image, the monochrome image is transferred from the intermediate transfer belt11to the sheet of recording paper and fixed to the sheet of recording paper.

Furthermore, when carrying out printing not only on the front side of the sheet of recording paper but on both sides, after the image on the front side of the sheet of recording paper is fixed by the fixing apparatus3and while transport rollers4-3of the paper transport apparatus4are transporting the sheet of recording paper, the transport rollers4-3can be made to stop and then rotate in reverse. The front and back of the sheet of recording paper are reversed by passing through a reverse path4rof the paper transport apparatus4, then the sheet of recording paper is guided to the registration roller8and an image is recorded and fixed on the back side of the sheet of recording paper in the same way as the front side of the sheet of recording paper, after which the sheet of recording paper is discharged to the discharge tray35.

<Description of Basic Configuration of Fixing Apparatus3>

Next, a basic configuration of the fixing apparatus3of the present embodiment is described.FIG. 2is a transverse cross-sectional view showing a vertical cross section of the heat roller3aand the pressure roller3bof the fixing apparatus3, andFIG. 3is a longitudinal cross-sectional view showing a horizontal cross section of the heat roller3aof the fixing apparatus3.

In the fixing apparatus3of the present embodiment, the heat roller3aand the pressure roller3bare axially supported to be freely rotatable and a nip area N is formed to sandwich the sheet of recording paper between the rollers3aand3bby pressing the rollers3aand3bagainst each other. Then, when one of the heat roller3aand the pressure roller3bis driven to rotate, the other follows and rotates, thereby introducing the sheet of recording paper to the nip area N between the rollers3aand3b, and pressure and heat are applied to the sheet of recording paper by the rollers3aand3b. Thus, as described earlier, the toner of the colors on the sheet of recording paper is thermally melted and mixed so that the toner image of the colors is fixed to the sheet of recording paper as a color image.

Furthermore, by pushing cleaning rollers71and71against the surfaces of the heat roller3aand the pressure roller3brespectively, the toner, paper dust and the like adhering to the surfaces of the rollers3aand3bare removed.

Further still, by pushing paper separating claws72and72against the surfaces of the heat roller3aand the pressure roller3brespectively, the sheet of recording paper is peeled off from the surfaces of the rollers3aand3b, which prevents sheets of recording paper from winding onto the rollers3aand3b.

The heat roller3ais a component in which a circumferential surface of a metal cylinder51is covered by an elastic layer52made of SiO2rubber. Flanges51aare provided at both edges of the cylinder51respectively and the elastic layer52is arranged between the flanges51a. Furthermore, pipe shaped rotating shafts51bare provided protruding from a center of the flanges51aof the cylinder51respectively, with these rotating shafts51bbeing axially supported to be rotatable. Further still, a main heater53and a sub-heater54are arranged along a longitudinal direction of the heat roller3ainside the cylinder51, and the heat roller3ais heated by heat produced by the main heater53or the sub-heater54. Terminals53aat the ends of the main heater53and terminals54aat the ends of the sub-heater54are connected through the rotating shafts51bto a control portion55.

As with the heat roller3a, the pressure roller3bis also a component in which a circumferential surface of a metal cylinder56is covered by an elastic layer57made of SiO2rubber. As with the cylinder51of the heat roller3a, the cylinder56is provided with flanges56aat its edges and pipe shaped rotating shafts56b, with the rotating shafts56bbeing axially supported to be rotatable. Furthermore, a single heater58is arranged along a longitudinal direction of the pressure roller3binside the cylinder56, and the pressure roller3bis heated by heat produced by the heater58. Terminals58aat the ends of the heater58are also connected through the rotating shafts56bof the cylinder56to the control portion55. The heater58heats substantially the entire pressure roller3band uniformly raises the surface temperature of the pressure roller3b. It should be noted that the heater58of the pressure roller3bis sometimes not provided depending on such factors as the specifications of the image forming apparatus.

The foregoing has been a description of the basic configuration of the fixing apparatus3.

Description of Fixing Apparatus According to Present Embodiment

With the present embodiment in the above-described configuration, the pressing force of the nip area N, which is a contact area between the heat roller3aand the pressure roller3b, can be varied between color processing mode to carry out color printing and monochrome processing mode to carry out monochrome printing. Specifically, at the time of color processing mode, the pressing force of the nip area N is lowered to narrow the nip width, and at the time of monochrome processing mode, the pressing force of the nip area N is raised to widen the nip width. Here, in the present embodiment, the position of the heat roller3ais fixed and the pressure roller3bis made to approach or move away from the heat roller3a, thereby varying the pressing force and varying the nip width. Furthermore, in the present embodiment, a positional relationship between the heat roller3aand the pressure roller3bat the time of monochrome processing mode is set as a normal position (that is, a home position), and a pressure application mechanism portion100is provided by which a movement is performed so that the pressure roller3bis moved away from the heat roller3aby a predetermined distance with respect to this positional relationship at the time of color processing mode, and after color processing mode is finished, a movement is performed so that the pressure roller3bis made to approach the heat roller3aagain and return to the home position. It should be noted that the pressure application mechanism portion100is described in detail later.

Here, two movement techniques are shown inFIG. 4(a) andFIG. 4(b) as movement techniques for causing the pressure roller3bto approach or move away from the heat roller3a.

That is, a rotational center Ra of the rotating shaft of the heat roller3aand a rotational center Rb of the rotating shaft of the pressure roller3bare arranged along a direction (hereinafter referred to as “horizontal direction”) X that is perpendicular to a paper transport direction Y, and inFIG. 4(a), the pressure roller3bis caused to approach and move away from the heat roller3aby causing the rotational center Rb of the pressure roller3bto move in a lateral direction along the horizontal direction X. InFIG. 4(a), a position (rotational center Rb) of the pressure roller3bshown by a solid line is the home position corresponding to the monochrome processing mode and the width of the nip area N at this time is shown as L1. In contrast to this, a position (rotational center Rb′) of the pressure roller3bshown by a dashed double-dotted line (imaginary line) is the position corresponding to the color processing mode and the width of the nip area N at this time is shown as L2. That is, when the width L1of the nip area N at the time of the monochrome processing mode is set to the width of the nip area N for normal times, the width L2of the nip area N at the time of the color processing mode is a narrower width than L1.

Here, in the movement technique shown inFIG. 4(a), since the pressure roller3bmoves along the horizontal direction X, the position of the nip area N is also slightly displaced laterally with respect to the paper transport direction Y. Consequently, insertion points n1and n1′ of the leading edge portion of the sheet of recording paper to the nip area N change between the color processing mode and the monochrome processing mode.

For this reason, when for example the insertion point of the leading edge portion of the sheet of recording paper to the nip area N is set to (n1) so as to be appropriate during the monochrome processing mode, the insertion point n1′ to the nip area N during the color processing mode becomes farther, and therefore the distance from where the leading edge portion of the sheet of recording paper contacts the surface of the heat roller3aon the left side until it is guided to the insertion point n1′ becomes longer, such that during this time there is a possibility of bending of the leading edge portion of the sheet of recording paper and positional displacement from the insertion point n1′ of the nip area N. On the other hand, when the insertion point of the leading edge portion of the sheet of recording paper to the nip area N is set to (n1′) so as to be appropriate during the color processing mode, the insertion point n1to the nip area N during the monochrome processing mode becomes closer, and therefore the distance from where the leading edge portion of the sheet of recording paper contacts the surface of the heat roller3aon the left side until it is guided to the insertion point n1becomes shorter, such that there is a possibility that the leading edge portion of the sheet of recording paper is not guided smoothly to the insertion point n1of the nip area N, and an undulating movement of the sheet of recording paper due to collision at the insertion point n1affects the preceding transfer process and causes transfer displacement.

A movement technique that addresses these problems is shown inFIG. 4(b).

InFIG. 4(b), to ensure that an insertion point n0of the nip area N does not move, the pressure roller3bis made to approach the heat roller3aby rotating the rotational center from Rb′ to Rb and the pressure roller3bis made to move away from the heat roller3aby rotating the rotational center from Rb to Rb′. In this case, inFIG. 4(b), a position (rotational center Rb) of the pressure roller3bshown by a solid line is the home position corresponding to the monochrome processing mode and the width of the nip area N at this time is shown as L1. In contrast to this, a position (rotational center Rb′) of the pressure roller3bshown by a dashed double-dotted line is the position corresponding to the color processing mode and the width of the nip area N at this time is shown as L2. That is, when the width L1of the nip area N at the time of the monochrome processing mode is set to the width of the nip area N for normal times, the width L2of the nip area N at the time of the color processing mode is a narrower width than L1.

Here, in the movement technique shown inFIG. 4(b), since the pressure roller3bis subjected to a rotational movement centered on the insertion point n0of the nip area N, the position of the insertion point n0of the nip area N is always kept constant with respect to the paper transport direction Y. Thus, the above-described problems that can occur due to the movement operation shown inFIG. 4(a) do not occur. That is, the leading edge portion of the sheet of recording paper can be made to enter the nip area N smoothly regardless of the processing mode.

FIG. 5toFIG. 7(b) show a detailed configuration of the pressure application mechanism portion100for achieving the movement technique shown inFIG. 4(b).FIG. 5is an exploded perspective view.FIG. 6is a perspective view showing an assembled state.FIGS. 7(a) and7(b) are schematic views as viewed from a lateral plane. It should be noted that the pressure application mechanism portions100are arranged at the rotating shafts51band56brespectively of the left and right sides of the heat roller3aand the pressure roller3b, but only one side is shown inFIG. 6.

As shown inFIG. 5, the pressure application mechanism portion100is broadly divided into a pressure-varying link structure101provided so as to be movable in lateral directions R1and R2along an upper end portion201aof a device frame201, a drive shaft131having an eccentric cam133for moving the pressure-varying link structure101in the lateral directions R1and R2, a shaft holding structure151provided so as to be rotatable coupled to the pressure-varying link structure101, and a coil spring171for applying a biasing force to the shaft holding structure151in a rightward direction R1along the upper end portion201aof the device frame201. First, these structural members are described separately.

The drive shaft131is provided with a large-diameter first drive shaft132that is coupled at a back end portion to a step motor or the like not shown in the drawings, the eccentric cam133integrally formed at a front end portion (front side inFIG. 5) of the first drive shaft132, and a small-diameter second drive shaft134that protrudes forward (front side inFIG. 5) from a lateral surface of the eccentric cam133.

Provided in a vicinity of the upper end portion201aon the left side of the device frame201is a first support hole202into which the first drive shaft132is rotatably inserted and supported, and further left from the first support hole202is formed a cutout portion203having a predetermined width and predetermined depth from the upper end portion201a. Furthermore, provided in a central area of the device frame201is a second support hole204into which the rotating shaft51bof the heat roller3ais rotatably inserted and supported, and moreover in a right side vicinity of the second support hole204is provided an opening portion205for enabling the rotating shaft56bof the pressure roller3bto be movably inserted.

Furthermore, a pressure plate210, which has been formed as a square-cornered “C” shape such that its upper area and lower area are open on the right side, is integrally fastened at a right end of the device frame201, and a cutout portion211is formed in a central area of the pressure plate210so as to connect to the opening portion205formed in the device frame201. Further still, shaft insertion holes215are formed in the square-cornered “C” shaped lower area of the pressure plate210into which a connecting shaft (male side shaft and female side shaft)181is inserted and supported for rotatably supporting the shaft holding structure151, which is described later.

The pressure-varying link structure101is formed by a front side plate102, an upper surface plate103, and a rear side plate104to make a downward facing “C” shape in transverse cross section and is arranged in a fitted state straddling the upper end portion201aof the device frame201. A cylinder shaped contact structure105is provided between the front side plate102and the rear side plate104on the left end side of the pressure-varying link structure101to suppress movement of the pressure-varying link structure101by being in constant contact with the eccentric cam133. Furthermore, laterally elongated holes106are formed in the front side plate102and the rear side plate104further rightward from the contact structure105, which is on the left end side of the pressure-varying link structure101, and an opening portion107is formed on the upper surface plate103. The opening portion107is provided to secure a clearance region of the eccentric cam133so as to not hinder the rotational movement of the eccentric cam133. Further still, shaft insertion holes108are provided at the front side plate102and the rear side plate104on the right end side of the pressure-varying link structure101into which a connecting shaft (male side shaft and female side shaft)182is inserted and supported for connecting to the shaft holding structure151.

The shaft holding structure151is formed by a front side plate152, a pressure receiving plate153, and a rear side plate154to make a square-cornered “C” shape and an opening portion thereof is arranged so as to be in opposition to an opening portion of the pressure plate210. A cutout portion (shaft holding portion)155formed in a substantial half circular arc is provided on opening side edge portions of the front side plate152and the rear side plate154of the shaft holding structure151to hold the rotating shaft56bof the pressure roller3b. Furthermore, shaft insertion holes156are provided at the front side plate152and the rear side plate154on the upper end portion of the shaft holding structure151into which the connecting shaft182is inserted and supported for connecting to the pressure-varying link structure101. Further still, shaft insertion holes157are provided at the front side plate152and the rear side plate154on the lower end portion of the shaft holding structure151into which the connecting shaft181is inserted and supported for rotatably supporting the pressure plate210.

In the pressure application mechanism portion100of the present embodiment in the above-described structure, the first drive shaft132is rotatably inserted and supported in the first support hole202of the device frame201, and in this state the pressure-varying link structure101fits so as to contain the eccentric cam133of the drive shaft131and to straddle the upper end portion201aof the device frame201. At this time, the second drive shaft134of the drive shaft131is inserted into the two elongated holes106and106of the pressure-varying link structure101and the eccentric cam133is arranged so as to touch the contact structure105. It should be noted that the contact structure105fits into a location of the cutout portion203provided in the device frame201. Further still, the positions of the shaft insertion holes108of the pressure-varying link structure101and the shaft insertion holes156of the shaft holding structure151are aligned and rotatably linked by the connecting shaft182. Next, the positions of the shaft insertion holes215of the pressure plate210and the shaft insertion holes157of the shaft holding structure151are aligned and rotatably linked by the connecting shaft181. Then, the coil spring171is mounted in a compressed state between the pressure plate210and the pressure receiving plate153of the shaft holding structure151. Further still, when the rotating shaft51bof the heat roller3ais inserted and held in the second support hole204of the device frame201and the rotating shaft56bof the pressure roller3bis inserted and held in the opening portion205of the device frame201and the cutout portion (shaft holding portion)155of the shaft holding structure151, the pressure application mechanism portion100is constructed having a structure illustrated inFIG. 6.

It should be noted that it is in fact impossible to attach the pressure-varying link structure101as it is, but by dividing the pressure-varying link structure101into two front and rear parts for example and combining the two parts into one whole after mounting, the above-described structure can be obtained. Also, the order in which each member is attached to the device frame201is not necessarily the above-described order, and attachments can be carried out in the most efficient order for the manufacturing process.

With this structure, the shaft holding structure151is configured to be always biased by the coil spring171toward the right side in the drawing and the biasing force is impeded by contacting the eccentric cam133of the drive shaft131and the contact structure105provided in the pressure-varying link structure101. Accordingly, the contact position of the contact structure105to the eccentric cam133is changed by driving the drive shaft131to rotate the eccentric cam133, and the pressure-varying link structure101moves in the R1or R2direction shown inFIG. 5accompanying this change. Then, accompanying the movement of the pressure-varying link structure101, the shaft holding structure151rotates centered on the axis of the connecting shaft181, a result of which is that a rotational movement is carried out in which the rotating shaft56bof the pressure roller3bbeing held by the shaft holding structure151approaches and moves away from the rotating shafts51bof the heat roller3a.FIGS. 7(a) and7(b) show conditions at this time. It should be noted that a ball bearing type shaft bearing structure may be used for the outer circumferential surface of the contact structure105so that the rotational movements of the eccentric cam133can be carried out smoothly.

FIG. 7(a) shows a condition in which the largest swollen portion of the eccentric cam133is contacting the contact structure105and shows a condition in which the pressure-varying link structure101has moved farthest to the left side. That is, a condition is shown in which the pressure roller3bis pressed against the heat roller3athe most strongly, the condition at this time being a condition shown by the solid line inFIG. 4(b). In other words, this is a condition in which the nip width of the nip area N is L1.

On the other hand,FIG. 7(b) shows a condition in which the eccentric cam133ofFIG. 7(a) has rotated 180°, that is, a condition in which the largest swollen portion of the eccentric cam133is positioned on an opposite side to the contact structure105, and a condition in which the pressure-varying link structure101has moved farthest to the right side. That is, a condition is shown in which the pressure roller3bis farthest apart from the heat roller3a, the condition at this time being a condition shown by the dashed double-dotted line inFIG. 4(b). In other words, this is a condition in which the nip width of the nip area N is L2.

It should be noted that in order to cause rotation of the rotational center Rb (Rb′) of the pressure roller3bsuch that the insertion point n0of the nip area N does not move as shown inFIG. 4(b), giving consideration to the paper transport direction, adjustment should be carried out of the positional relationships of three points, namely the rotational center Ra of the heat roller3a, the rotational center Rb (Rb′) of the pressure roller3b, and the rotational center (axis of the connecting shaft181) of the shaft holding structure151.

Furthermore, in the above-described embodiment, by arranging the pressure-varying link structure101straddling the upper end portion201aof the device frame201, a configuration is achieved that prevents longitudinal wobbling when the pressure-varying link structure101moves in a lateral direction, but it is not absolutely necessary for the pressure-varying link structure101to be arranged straddling the device frame201, and may be arranged along one side (the front side inFIG. 5) of the device frame201.

Furthermore, in regard to the structure of the pressure application mechanism portion100for achieving the movement technique shown inFIG. 4(a), this can be easily achieved by employing the pressure application mechanism portion100shown inFIGS. 5 to 7(b). That is, this structure can be achieved by arranging the drive shaft131, the pressure-varying link structure101, and the coil spring171arranged on the upper end portion201aof the device frame201also on a lower end portion side, and by making the connecting structure on the lower side of the shaft holding structure151be the same structure as a connecting structure on the upper side.

The present invention can be embodied and practiced in other different forms without departing from the gist and essential characteristics thereof. Therefore, the above-described embodiments are considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. All variations and modifications falling within the equivalency range of the appended claims are intended to be embraced therein.