Sheet cooling apparatus and image forming apparatus

A sheet cooling apparatus cools a sheet while conveying the sheet having passed through a fixing device for fixing an unfixed toner image formed on the sheet by heating the toner image. A second suspension member is arranged so as to press a second endless belt on a circumferential face of a first cooling roller through a first endless belt, a first suspension member is arranged so as to press the first endless belt on a circumferential face of a second cooling roller through the second endless belt, and a curved sheet conveying path is formed between the first endless belt and the second endless belt.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a sheet cooling apparatus used for cooling a sheet, which is formed from paper fibers or the like, that is a target for heating, pressing, and fixing an unfixed toner image in an image forming apparatus such as a copying machine, a printer, or a facsimile machine.

Description of the Related Art

Conventionally, in image forming apparatuses such as electro-photographic apparatuses and electrostatic recording apparatuses, an image is formed on a sheet by forming a toner image on the sheet as a recording material and fixing the toner image using a fixing device by heating and pressing the toner image. As fixing devices used in such image forming apparatuses, a roller fixing system is employed in which a pressing nip portion (fixing nip portion) is formed by pressing a pressure roller to a fixing roller having an internal heater and performing fixing by performing heating and pressing in the pressing nip portion.

In a fixing device of the roller fixing system, heat is applied to toner and a sheet, and accordingly, moisture contained in the sheet evaporates in the pressing nip portion and after passing the pressing nip portion. Then, ripples and curls occur due to a change in the amount of moisture of the sheet and stress applied to the sheet.

When a sheet is viewed in the level of fibers, the sheet is formed by entangling short fibers with each other, moisture is contained inside the fibers or between the fibers, and the fibers and water generate hydrogen bonding. In a fixing process, when heat is applied to the sheet, the moisture included inside the sheet evaporates, and accordingly, the hydrogen bonding occurs between the fibers, whereby the sheet is deformed. When the sheet is left around, the sheet absorbs moisture from the environment, and the hydrogen bonding between the fibers is broken again. However, moisture is not permeated between some fibers, whereby the deformation is maintained.

As a pattern of the deformation, there are deformations according to a difference in the expansion and contraction between the front and rear sides of a sheet and deformation according to a difference in the expansion and contraction between a center portion and an end portion of a sheet. In accordance with such deformation, ripples and curls occur in the sheet.

In order to solve such a problem, a configuration is disclosed in which the sheet is cooled as below.

In Japanese Patent Laid-Open No. 2009-161347, a bending portion bending in a direction opposite to the bending of a sheet on which an image is fixed by a fixing portion in the conveying direction is included. In addition, a cooling member that cools a sheet conveyed by a belt member through the belt member in an area including the bending portion and a pressing member that presses a sheet to the bending portion side of the cooling member are included.

In Japanese Patent Laid-Open No. 2009-175260, a configuration is disclosed in which an endless belt member having good heat conductivity is stretched over belt cooling rollers that are aligned from a fixing device in the conveying direction. Then, a sheet heated by the fixing device is brought into contact with the endless belt member stretched between the belt rollers so as to be cooled, and the endless belt member heated by the sheet is cooled by the belt cooling rollers.

However, among the above-described conventional technologies, in the technology disclosed in Japanese Patent Laid-Open No. 2009-161347, there is concern that tension of the belt member may increase, and deterioration due to abrasion caused by a contact load of the belt member for the cooling member may become serious.

In addition, in the technology disclosed in Japanese Patent Laid-Open No. 2009-175260, since the contact area between the endless belt member and the belt cooling rollers is small, there is concern that the heat transfer to the endless belt member is lowered in accordance with an increase in the number of continuously passing sheets.

SUMMARY OF THE INVENTION

It is desirable to improve the durability by decreasing the abrasion of a cooling belt and members brought into contact with the inner face of the cooling belt and to realize improvement of the cooling ability.

According to the invention, there is provided a sheet cooling apparatus that cools a sheet while conveying the sheet passing through a fixing device fixing an unfixed toner image formed on the sheet by heating the toner image. The sheet cooling apparatus includes: a first endless belt that is suspended on a first cooling roller and a first suspension member arranged downstream of the first cooling roller in a conveying direction; and a second endless belt that is suspended on a second cooling roller arranged downstream of the first cooling roller in the conveying direction and a second suspension member arranged upstream of the second cooling roller in the conveying direction; wherein the second suspension member is arranged so as to press the second endless belt on a circumferential face of the first cooling roller through the first endless belt, the first suspension member is arranged so as to press the first endless belt on a circumferential face of the second cooling roller through the second endless belt, and a curved sheet conveying path is formed between the first endless belt and the second endless belt.

According to the invention, by using the cooling roller, the conveying resistance due to friction with a belt can be markedly lower than that of the fixed type, and accordingly, the belts and the sheet can be conveyed in a stable manner, and the driving load can be reduced.

In addition, by forming the curved conveying path bent in the shape of “S” along the circumferential face of the cooling roller, the belt and each cooling roller can be brought into contact with each other in the entire area of the sheet conveying path, whereby the cooling efficiency for the sheet can be improved.

Furthermore, by forming the curved conveying path bent in the shape of “S” other than a conveying path having an approximately linear shape, the miniaturization of the whole apparatus can be achieved, and the curl and the ripple can be reduced by acquiring a curl correction effect for the sheet.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, exemplary embodiments of the invention will be described in detail as examples with reference to the drawings. However, the sizes, the materials, the shapes, and the relative arrangements of constituent components described in the following embodiments should be appropriately changed in accordance with the configuration and various conditions of devices and apparatuses to which the invention is applied. Accordingly, unless there is specific description, the embodiments are not for purposes of limiting the scope of the invention thereto.

First Embodiment

A sheet cooling apparatus and an image forming apparatus including the sheet cooling apparatus will be described with reference toFIGS. 1 to 12.

FIG. 2is a cross-sectional view of a color electro-photographic printer500as an example of the image forming apparatus according to this embodiment and is a cross-sectional view taken along the conveying direction of a sheet. In this embodiment, the color electro-photographic printer will be simply referred to as a “printer”. On a sheet as a recording material, a toner image is formed. As specific examples of the sheet, there are plain paper, a sheet made from a resin that is a substitute for the plain paper, a cardboard, and an overhead projector sheet.

The printer500illustrated inFIG. 2includes image forming portions510of colors Y (yellow), M (magenta), C (cyan), and Bk (black). In the image forming portions510, toner images of each color to be formed on a sheet are formed. In each image forming portion510, a photosensitive drum511as an image bearing member is electrically charged by a charging roller512in advance. Thereafter, a latent image is formed on the photosensitive drum511by a laser scanner513. The latent image is formed as a toner image by a development device514. The toner images formed on the photosensitive drum511are transferred to an intermediate transfer belt531as an intermediate transfer member in a sequentially superimposed manner.

Sheets P as recording materials are sent from a sheet cassette520one by one and are conveyed to a pair of registration rollers523. The pair of registration rollers523receives the sheet P once and, in a case where the sheet P is fed on a skew, the skew feeding is immediately corrected. Then, the pair of registration rollers523sends the sheets P between the intermediate transfer belt531and a secondary transfer roller535in synchronization with a toner image formed on the intermediate transfer belt531. The color toner images formed on the intermediate transfer belt531are transferred together to the sheet P by the secondary transfer roller535as a transfer portion.

Thereafter, the unfixed toner images T formed on the sheet P are fixed to the sheet P by being heated and pressed by a fixing device100. After passing through the fixing device100, the sheet P is cooled while being conveyed to the inside of a cooling apparatus101as a sheet cooling apparatus and is discharged to a discharge tray565with the face up (the toner image is disposed on the upper side).

The fixing device100and the cooling apparatus101as a sheet cooling apparatus will be described with reference toFIGS. 1, 3, and 4. First, the fixing device100will be described with reference toFIG. 1, and then, the cooling apparatus101will be described with reference toFIGS. 1, 3, and 4.

As illustrated inFIG. 1, the fixing device100includes a fixing roller110as a fixing member and a pressure roller111as a pressure member. The fixing roller110applies heat emitted from an internal halogen heater (not illustrated in the figure) to the toner image T formed on the sheet P and conveys the sheet P cooperatively with a pressure roller111. The fixing roller110, for example, includes a metal core formed from an aluminum cylindrical pipe having an outer diameter of 56 mm and an inner diameter of 50 mm, and a halogen heater is built inside the metal core. In addition, the fixing roller110is acquired by coating the surface of the metal core with an elastic layer formed from silicon rubber, for example, having a thickness of 2 mm and a hardness (ASKER C) of 45° and further coating the surface layer of the elastic layer with a PFA or PTFE heat-resistant toner parting layer.

The pressure roller111conveys the sheet P cooperatively with the fixing roller110. The pressure roller111includes a metal core formed by an aluminum cylindrical pipe, for example, having an outer diameter of 56 mm and an inner diameter of 50 mm. In addition, the pressure roller111is acquired by coating the surface of the metal core with an elastic layer formed from silicon rubber, for example, having a thickness of 2 mm and a hardness (ASKER C) of 45° and further coating the surface layer of the elastic layer with a PFA or PTFE heat-resistant toner parting layer.

The fixing device100forms a fixing nip (pressing nip) N illustrated inFIG. 1by using the fixing roller110and the pressure roller111.

The sheet P that is conveyed to the fixing device100by the photosensitive drum511and the transfer roller535enters the fixing nip N of the fixing roller110and the pressure roller111. The fixing roller110is heated and pressed inside the fixing nip N formed by the fixing roller110and the pressure roller111, whereby the unfixed toner image T is fixed to the sheet P. The sheet P on which the image is fixed, as illustrated inFIG. 1, is guided to an area between an upper discharge guide501and a lower discharge guide502.

The sheet P discharged from the fixing device100is guided to the cooling apparatus101by the guides501and502and is cooled so as to remove the heat applied by the fixing device100while being conveyed by the cooling apparatus101.

As illustrated inFIG. 1, the sheet P guided to an area between the upper discharge guide501and the lower discharge guide502is conveyed to an area between cooling belts302and202used for nipping and conveying the sheet P in the cooling apparatus101while contacting the front surface and the rear surface of the sheet P.

The cooling belts302and202are endless belts (endless belt members) that are brought into contact with the face of a sheet and convey the sheet. The cooling belts302and202may be formed from a material having superior thermal conductivity and may be formed from a material such as a polyimide film, a nickel electroformed film, or a polyethylene film that can form a thin plate.

The cooling belt202as a second endless belt performs frictional conveyance while being brought into contact with the rear surface of the sheet P. This cooling belt202, as illustrated inFIG. 1, is stretched over the outer circumferences of a cooling roller201as a second cooling roller and belt pressure rollers204and205as second suspension members and a tension roller203. Tension is applied to the cooling belt202by the tension roller203.

The cooling roller201is arranged on the downstream of a cooling roller301as a first cooling roller to be described later in the conveying direction. The belt pressure rollers204and205as the second suspension members and the tension roller203are arranged on the upstream of the cooling roller201in the conveying direction.

The cooling belt202, the cooling roller201, the belt pressure rollers204and205, and the tension roller203, as illustrated inFIG. 3, configure an independent cooling belt unit200.

The cooling roller201, as illustrated inFIGS. 3 to 5, is supported to be rotatable through a cooling roller bearing211by a front side plate210and a rear side plate220that configure the frame of the cooling belt unit200. The front side plate210and the rear side plate220are connected and fixed to both end portions of a stay206, thereby forming the frame of the cooling belt unit200. An upper end portion206aof the stay206, as illustrated inFIG. 1, serves also as a discharge guide that guides the lower face side of the sheet P discharged from an exit portion C of a conveyance nip of the cooling apparatus101.

As illustrated inFIGS. 3, 6A, and 7A, a cooling roller driving gear201G is fixed to an outer side adjacent to one cooling roller bearing211of the cooling roller201. In addition, a cooling duct291used for guiding cooling air to a cooling fan290from an internal cavity of the cooling roller201is disposed on a face that is located on the outer side further adjacent to the cooling roller driving gear201G and on the outer side of the rear side plate220. The cooling duct291is connected and fixed to the cooling roller driving gear201G in a non-contact manner on the same axis so as not to block rotation of the cooling roller201. A cooling fan290is fixed to a further outer face of the cooling duct291. In other words, in order to cause cooling air to flow to the inside the cooling roller201, the cooling fan290is disposed in one end portion in the direction of the rotation shaft of the cooling roller201.

On the outer face of the rear side plate220, as illustrated inFIGS. 3 and 5, a driving input gear292is supported to be rotatable so as to be engaged with the cooling roller driving gear201G and is supplied with a driving force from a driving motor that is a driving source not illustrated in the figure.

The belt pressure rollers204and205, as illustrated inFIGS. 3 and 5, are supported to be rotatable through bearings213and223by pressure arms212and222. In addition, the pressure arms212and222are supported to be swingable by pressure arm supporting shafts210band220bthat are integrally formed on the side faces of the front side plate210and the rear side plate220.

Between pressure spring bearing faces210cand220cintegrally formed on the outer faces of the front side plate210and the rear side plate220and the pressure arms212and222, pressure springs214and224of the compression coil type are inserted, and the pressure springs214and224bias the pressure arms212and222in a direction pushing them up. Accordingly, the belt pressure rollers204and205supported by the pressure arms212and222among a plurality of second suspension members are pressed to the cooling roller301through the cooling belts302and202. Here, although a configuration has been described as an example in which two rollers204and205among the plurality of second suspension members, which are disposed, are pressed to the cooling roller301, the configuration is not limited thereto, and at least one suspension member may be pressed to the cooling roller.

On the outer faces of the front side plate210and the rear side plate220, belt tensioners215and225used for applying tension to the cooling belt202are arranged. Tension roller bearings218and228supporting both end portions of the tension roller203to be rotatable are supported also by hollow inner walls of tensioner holders216and226having hollow inner portions in the shape of a rectangle so as to be slidable in the horizontal direction inFIG. 1. In addition, the other ends of the tension springs217and227of the pulling coil type having one ends hooked into one hollow inner walls of the tensioner holders216and226are hooked into the tension roller bearings218and228, and the tension roller bearings218and228are pulled and biased. Accordingly, as illustrated inFIGS. 1 and 5, in a state in which the cooling belt units200and300are contacted and pressed through the cooling belts202and302, the tension roller203applies tension to the cooling belt202while being supported to be rotatable.

In addition, on the outer faces of the front side plate210and the rear side plate220used for supporting the tension roller203, unit supporting shafts210dand220dused for supporting the front side plate310and the rear side plate320to be swingable are integrally formed. The front side plate310and the rear side plate320configure a frame of the cooling belt unit300that is the other cooling belt unit including the cooling belt302on the inside thereof.

Furthermore, in uppermost portions of the outer faces of the front side plate210and the rear side plate220, unit pressure spring engaging portions210aand220a, as illustrated inFIG. 3, are formed integrally with the front side plate210and the rear side plate220. The unit pressure spring engaging portions210aand220aare disposed so as to hook lower one ends of the unit pressure springs280and281of the compression coil type used for pressing the cooling belt units200and300illustrated inFIG. 5toward each other.

The cooling belt302as a first endless belt performs frictional conveyance while being brought into contact with the front surface of the sheet P. This cooling belt302, as illustrated inFIG. 1, is stretched over the outer circumferences of a cooling roller301as a first cooling roller and belt pressure rollers303and304as first suspension members and a tension roller305. Tension is applied to the cooling roller301by the tension roller305.

The cooling roller301is arranged on the upstream of the cooling roller201in the conveying direction as the second cooling roller described above. The belt pressure rollers303and304as the first suspension members and the tension roller305are arranged on the downstream of the cooling roller301in the conveying direction.

The cooling belt302, the cooling roller301, the belt pressure rollers303and304, and the tension roller305, as illustrated inFIG. 4, configure an independent cooling belt unit300.

The cooling roller301, as illustrated inFIG. 4, is supported to be rotatable through a cooling roller bearing311by a front side plate310and a rear side plate320that configure the frame of the cooling belt unit300. The front side plate310and the rear side plate320are connected and fixed to both end portions of a stay306, thereby forming the frame of the cooling belt unit300.

As illustrated inFIGS. 4, 6B, and 7B, a cooling duct391used for guiding cooling air to a cooling fan390from an internal cavity of the cooling roller301is disposed on an outer a face of the rear side plate320, is connected and fixed on the same shaft in a non-contact manner so as not to block rotation of the cooling roller301. A cooling fan390is fixed to a further outer face of the cooling duct391. In other words, in order to allow cooling air to flow to the inside the cooling roller301, the cooling fan390is disposed in one end portion in the direction of the rotation shaft of the cooling roller301.

The belt pressure rollers303and304, as illustrated inFIGS. 4 and 5, are supported to be rotatable through bearings313and323by pressure arms312and322. In addition, the pressure arms312and322are supported to be swingable by pressure arm supporting shafts310band320bthat are integrally formed on the side faces of the front side plate310and the rear side plate320.

Between pressure spring bearing faces310cand320cintegrally formed on the outer faces of the front side plate310and the rear side plate320and the pressure arms312and322, pressure springs314and324of the compression coil type are inserted, and the pressure springs314and324bias the pressure arms312and322in a direction pushing them down. Accordingly, the belt pressure rollers303and304supported by the pressure arms312and322among a plurality of first suspension members are pressed to the cooling roller201through the cooling belts302and202. Here, although a configuration has been described as an example in which two rollers303and304among the plurality of first suspension members, which are disposed, are pressed to the cooling roller201, the configuration is not limited thereto, and at least one suspension member may be pressed to the cooling roller.

On the outer faces of the front side plate310and the rear side plate320, belt tensioners315and325used for applying tension to the cooling belt302are arranged. Tension roller bearings318and328supporting both end portions of the tension roller305to be rotatable are supported also by hollow inner walls of tensioner holders316and326having hollow inner portions in the shape of a rectangle so as to be slidable in the horizontal direction inFIG. 1. In addition, the other ends of the tension springs317and327of the pulling coil type having one ends hooked into one hollow inner walls of the tensioner holders316and326are hooked into the tension roller bearings318and328, and the tension roller bearings318and328are pulled and biased. Accordingly, as illustrated inFIGS. 1 and 5, in a state in which the cooling belt units200and300are contacted and pressed through the cooling belts202and302, the tension roller305applies tension to the cooling belt302while being supported to be rotatable.

In addition, on the outer faces of the front side plate310and the rear side plate320used for supporting the tension roller305, bearings310dand320dused for allowing the front side plate310and the rear side plate320to be supported by the unit supporting shafts210dand220don the side faces of the front side plate210and the rear side plate220to be swingable are held. The front side plate210and the rear side plate220configure a frame of the cooling belt unit200that is the other cooling belt unit including the cooling belt202on the inside thereof.

Furthermore, in uppermost portions of the outer faces of the front side plate310and the rear side plate320, unit pressure spring engaging portions310aand320afor hooking upper one ends of the unit pressure springs280and281are integrally formed. The unit pressure springs280and281are disposed for pressing the cooling belt units200and300illustrated inFIGS. 4 and 5each other.

In the cooling belt units200and300, the unit supporting shafts210dand220ddisposed on the side faces of the front side plate210and the rear side plate220fit into the bearings310dand320dheld on the side faces of the front side plate310and the rear side plate320. The cooling rollers201and301contact each other at a point B illustrated inFIG. 1through the cooling belts202and302in accordance with biasing forces of the unit pressure springs280and281.

Accordingly, as illustrated inFIG. 1, after passing through a contact point A at which the cooling belts302and202first contact each other to a contact point B at which the cooling rollers201and301contact each other, the cooling belts302and202contact each other over a contact point C, at which separation is started, in the entire area (denoted by a thick line). Accordingly, when a sheet P is conveyed, the sheet P is conveyed in a frictional manner with the front surface and the rear surface of the sheet P being brought into surface contact with the cooling belts302and202. The heat of the sheet P is transferred to the cooling belts302and202in accordance with the surface contact and is radiated through the cooling rollers201and301.

In addition, the cooling rollers201and301that suspend the cooling belts202and302press each other, and a conveying path that is a sheet conveying path is bent in the shape of “S” so as to be arranged as denoted by a thick line inFIG. 1. More specifically, the belt pressure rollers205and204are arranged to contact the cooling belt202on the circumferential face of the cooling roller301, and the belt pressure rollers303and304are arranged to contact the cooling belt302on the circumferential face of the cooling roller201. From this, conveying paths bent in the shape of “S” that contact the cooling belts202and302from the circumferential face of the cooling roller301along the circumferential face of the cooling roller201can be formed. In addition, the cooling conveying paths bent in the shape of “S” is configured to have a path length that approximately matches a maximum length of a sheet that can be used by the image forming apparatus500. From this, the sheet P can be conveyed by the cooling belts202and302while the sheet P is brought into surface contact with the cooling belts202and302and the cooling rollers201and301over the entire area of the conveying paths. Accordingly, the cooling efficiency for the sheet can be markedly improved, and the productivity of the image forming apparatus can be improved.

The cooling rollers201and301are formed from members (here, aluminum members) having a high heat radiation effect and, as illustrated inFIGS. 1, 7A, and 7B, have hollow inner portions. In addition, in the cooling rollers201and301, a plurality of heat radiation fins201aand301ahaving a protruded shape is formed on the inner walls having hollow inner portions. The plurality of heat radiation fins201aand301ais formed in a spiral shape in the direction of the rotation shafts of the cooling rollers201and301. From this, the surface area of each one of the cooling rollers201and301is larger than that of the pipe-shape of the hollow inner wall, and accordingly, the heat radiation effect is high.

In addition, in the spaces of the hollow inner portions of the cooling rollers201and301, as illustrated inFIGS. 7A and 7B, cooling air is caused to flow by the cooling fans290and390, whereby the radiation of heat to the outside the cooling apparatus101is promoted.

Furthermore, the spiral shaped heat radiation fins201aand301acause cooling air to be generated inside in accordance with the rotation of the cooling rollers201and301. As illustrated inFIGS. 7A and 7B, the advancement directions of the spirals of the heat radiation fins201aand301aof the cooling rollers201and301are opposite to each other with respect to the shaft directions. Accordingly, although the rotation directions of the cooling rollers201and301are different from each other, the wind direction of cooling air generated by the heat radiation fins201aand301afollowing the rotation of the cooling rollers201and301coincides with the wind direction of the cooling air of the cooling fans290and390. As a result, the cooling airs generated by the cooling fans290and390flow without blocking each other.

The cooling roller201rotates in the counterclockwise direction as illustrated inFIG. 1as the cooling roller driving gear201G fixed to one end side thereof receives a driving transfer from the driving input gear292provided with a driving force from a driving motor that is a driving source not illustrated in the figure. From this, the cooling belt202is frictionally driven in the direction of an arrow illustrated inFIG. 1, and the belt pressure rollers204and205and the tension roller203are also driven to rotate. In addition, the cooling belt302is driven and conveyed in a direction of the arrow illustrated inFIG. 1in accordance with the frictional driving from the cooling belt202, whereby the belt pressure rollers303and304and the tension roller305are driven to rotate.

A path length from the contact point A to the contact point C that is a conveying path illustrated inFIG. 1formed by the cooling rollers201and301is configured to approximately coincide with a length of a sheet P having a maximum length (for example, a sheet of the A3 size) that can be used in the image forming apparatus500using the sheet cooling apparatus. From this, the cooling efficiency for the sheet P can be maximally maintained without inhibiting the miniaturization of the whole device. For example, since the A3 size is 297 mm×420 mm, the path length from the contact point A to the contact point C is preferably set to about 400 mm to 450 mm.

An experiment relating to sheet cooling was performed by the inventors of the invention under the condition that the set temperature of the surface layer of the fixing roller110was 180° C., the set temperature of the surface layer of the pressure roller111was 100° C., the ambient temperature was 23° C., and the ambient humidity was 50%. As an example of a specific experiment, under this condition, an experiment was performed in which a sheet P that was plain paper having a basis weight of about 70 to 80 g and an internal moisture content of about 6% is conveyed at an conveying speed of about 300 to 500 mm/sec. Then, an experiment result was acquired in which the sheet P right after passing through the fixing nip N was heated to a surface temperature of about 90° C., and the internal moisture content decreased to about 4%.

At this time, the sheet P heated and pressed inside the fixing nip portion N receives heat more from the fixing roller110having high temperature than from the pressure roller111, and fibers grow more on the upper face side of the sheet P that is the fixing roller110side than on the lower face side of the sheet P that is the pressure roller111side. From this, consequently, a curl formed in the lower direction (hereinafter, referred to as a downward curl) occurs in the sheet P. In addition, the moisture content near the end portion of the sheet P in the width direction decreases more, in which one side is not bound, and moisture movement from/to the air can easily occur, decreases more than in the center portion of the sheet P in the width direction in which the periphery is restrained by the fiber structure. Accordingly, since the fibers of the sheet P can easily grow, and, consequently, a phenomenon (hereinafter, referred to as a ripple) occurs in which the surface of the end portion of the sheet P is deformed in a shape having ripples in the vertical direction.

Under the above-described condition, for example, there are cases where the amount of the downward curl occurring in the front end portion and the rear end portion of the sheet P is 10 to 15 mm, and the height of the ripple in the end portion in the width direction is about 1.5 to 2 mm.

In the path from the contact point A to the contact point C (denoted by a thick line inFIG. 1) that is the cooling conveying path formed by the cooling belts302and202, the sheet P, first, passes a first bending path according to the curvature of the cooling roller301between the contact point A and the contact point B. Then, the sheet P continuously passes a second bending path according to the curvature of the cooling roller201between the contact point B and the contact point C. In the cooling conveying path from the contact point A to the contact point C, since it is immediately after the start of cooling, the sheet P passes the upward first bending path according to the curvature of the cooling roller301between the contact point A having relatively high temperature and the contact point B. Accordingly, in the first bending path, the sheet P is more effectively corrected for the curl thereof by the curl correction effect using the downward second bending path according to the curvature of the cooling roller201from the contact point B, which is a latter half of the cooling process, to the contact point C.

The path length from the contact point A to the contact point C is set to about 400 mm to 450 mm as described above. In such a case, in the sheet P passing the nip portion N and further passing an area between the upper discharge guide501and the lower discharge guide502, the upper face side and the lower face side are cooled to about 30° C. to 50° C. by the cooling rollers301and201, respectively, from the contact point A to the contact point C that is a cooling conveying path. Simultaneously, between the contact point A and the contact point B, the sheet P is corrected for the curl upwardly in accordance with the curvature of the upward bending path according to the curvature of the cooling roller301, and the amount of the downward curl occurring in the front end portion and the rear end portion of the sheet P is enhanced to be 0 to 5 mm.

In addition, an abrupt decrease in the amount of moisture can be prevented by cooling the sheet P, and, as a result, the amount of moisture inside the sheet is enhanced to about 4.5 to 5%, and the height of the ripple in the end portion of the sheet in the width direction can be enhanced to about 0.5 to 0.8 mm.

Here, each one of the cooling rollers201and301may be a cooling roller401having the configuration illustrated inFIGS. 8 and 9.

InFIGS. 8 and 9, a heat pipe402is a thermal uniformization member and is disposed inside the cooling roller401. As illustrated inFIG. 9, heat radiation fins402aformed from a material such as aluminum or stainless steel having relatively high heat transference are fixed to one end portion of the heat pipe402.

Inside the cooling roller401, a hollow portion used for causing cooling air to flow using the cooling fan490is formed, and a plurality of the heat pipes402holds the heat radiation fins402atoward the cooling fan490side. As above, by arranging the heat pipes that are thermal uniformization members inside the cooling roller401, for example, even in the case of continuous conveying sheets of the A4 size in the vertical direction, a temperature difference between the inside of the conveying area in the width direction of the cooling roller401and the outside of the area can decrease as much as possible. Accordingly, an advantage that it is further difficult to spoil the cooling performance can be expected.

Other than the above-described configuration, for example, a cooling roller601having the configuration as illustrated inFIGS. 10, 11, and 12may be used.

As illustrated inFIGS. 10, 11, and 12, a plurality of heat pipe rollers630is acquired by coating the surface layers of the heat pipes, which are thermal uniformization members, with a PFA heat-resistant toner parting layer and are disposed inside the cooling roller601. As illustrated inFIG. 11, heat radiation fins630aformed from a material such as aluminum or stainless steel having relatively high heat transference are fixed to one end portion of the heat pipe roller630.

Both end portions of the heat pipe roller630, as illustrated inFIGS. 10 and 11, are supported to be rotatable by the cooling ducts691and692through a bearing631such that the heat radiation fin630afaces the cooling fan690side. A plurality of duct holes is formed on the side faces of the cooling ducts691and692.

As illustrated inFIG. 12, the heat pipe roller630is arranged to be inscribed in the inner wall of the cooling roller601having the inside that is hollow in the shape of a circle. Accordingly, when the cooling roller601rotates, each heat pipe roller630is driven to rotate in accordance with friction with the inner wall face of the cooling roller601. Even in such a case, the same advantage can be acquired by using the cooling roller401.

As described above, by forming a heat sink structure and using the cooling rollers201and301that can rotate together with the conveying of the cooling belts202and302, the conveying resistance due to the friction with the cooling belts202and302can be markedly smaller than that of the fixing type. Accordingly, the cooling belts202and302and the sheet P can be stably conveyed, whereby the driving load and the power can be reduced.

In addition, the deterioration of the durability such as abrasion of the surfaces of the cooling rollers201and301due to sliding with the cooling belts202and302does hardly occurs, thereby heat transference between the surface of members such as the cooling rollers201and301and the cooling belts202and302is stabilized. Accordingly, the cooling performance and the reliability of the durability of the sheet cooling apparatus can be markedly improved.

As a result, for example, it is not particularly necessary to perform a surface treatment such as an alumite treatment for the surfaces of the cooling rollers201and301, which are formed from aluminum, so as to reduce the abrasion and deterioration. In addition, similarly, it is not particularly necessary to perform a surface treatment having low frictional resistance using a fluororesin system or the like for the surfaces of the cooling rollers201and301so as to reduce the sliding resistance and the abrasion and deterioration. Accordingly, the heat transference between the surface of members such as the cooling rollers201and301and the cooling belts202and302is not degraded, and the component cost does not increase. Therefore, for example, it is possible to revive good heat radiation/cooling capability of an aluminum material or the like. In addition, there hardly is a decrease in the strength due to the progress of the abrasion/degradation of the surfaces of the cooling belts202and302, and, for example, even in a case where a polyimide material is used, the thickness can be decreased, whereby the heat transference with the cooling rollers201and301can be improved.

In addition, as described above, by pressing the cooling rollers201and301that suspend the cooling belts202and302and pressing the cooling rollers201and301to opposing cooling rollers using the belt pressure roller, the shape of the conveying path is arranged to be bent in the shape of “S” as denoted by a thick line inFIG. 1. In addition, the cooling conveying path bent in the shape of “S” is configured to have a path length that approximately coincides with a maximum length of a sheet that can be used in the image forming apparatus500. Accordingly, the sheet P can be conveyed by the cooling belts202and302while the cooling belts202and302and the cooling rollers201and301rollers and the sheet P are brought into surface contact with each other over the entire area of the conveying path. Therefore, the cooling efficiency for a sheet can be markedly improved, and the productivity of the image forming apparatus can be improved.

Furthermore, by using a curved path bent in the shape of “S” other than a cooling conveying path having an approximately linear shape, the miniaturization of the whole apparatus can be achieved, and the curl and the ripple can be reduced by acquiring a curl correction effect for the sheet.

Second Embodiment

A cooling apparatus102as a sheet cooling apparatus will be described with reference toFIGS. 13A to 20. Description of the same portion as that of the above-described first embodiment will not be presented.

A cooling belt702as a second endless belt used for performing frictional conveying while being brought into contact with the rear face of the sheet P, a cooling roller701, and belt pressure rollers704and705as second suspension members, and a tension roller703configure an independent cooling belt unit700. The configuration of the cooling belt unit700is the same as that of the cooling belt unit200according to the above-described first embodiment, and thus, description thereof will not be presented.

A cooling belt802as a first endless belt performs frictional conveyance while being brought into contact with the front surface of the sheet P. This cooling belt802, as illustrated inFIGS. 13A and 13B, is stretched over the outer circumferences of a cooling roller801as a first cooling roller and belt pressure roller803and a tension roller804as first suspension members. Tension is applied to the cooling belt802by the tension roller804. The cooling belt802, the cooling roller801, the belt pressure roller803, and the tension roller804configure an independent cooling belt unit800.

The configuration of the cooling roller801is the same as that of the cooling roller301according to the first embodiment illustrated inFIGS. 7A and 7B, and a method for supporting the cooling roller801toward a front side plate810and a rear side plate820configuring the frame of the cooling belt unit800is the same as that of the first embodiment.

The belt pressure roller803, as illustrated inFIGS. 14, 15, 16, and 17, is supported to be rotatable through bearings813and823by pressure arms812and822. In addition, the pressure arms812and822are supported to be swingable by pressure arm supporting shafts810band820bformed integrally with the side faces of the front side plate810and the rear side plate820.

Between pressure spring bearing faces810cand820cintegrally formed on the outer faces of the front side plate810and the rear side plate820and the pressure arms812and822, pressure springs814and824of the compression coil type are inserted, and the pressure springs814and824bias the pressure arms812and822in a direction pushing them down. Accordingly, the belt pressure roller803supported by the pressure arms812and822is pressed to the opposing cooling roller701through the cooling belts802and702.

On the outer faces of the front side plate810and the rear side plate820, belt tensioners815and825used for applying tension to the cooling belt802are arranged. The belt tensioner825disposed on one side, as illustrated inFIG. 17, is arranged on the outer face of the rear side plate820. The belt tensioner815on a side opposite thereto is arranged so as have a shape symmetrical to the belt tensioner825of the rear side plate820on the outer face of the front side plate810as illustrated inFIG. 14. The belt tensioners815and825have a configuration in which tension roller bearings, of which roller bearing828of belt tensioner825is shown inFIG. 17and a corresponding roller bearing of belt tensioner815is not shown in the figures, support both end portions of the tension roller804to be rotatable are supported by hollow inner walls of tensioner holders816and826having hollow inner portions in the shape of a rectangle so as to be slidable. In addition, one of the ends of tension springs of the pulling coil type, of which tension spring827of belt tensioner825is shown inFIG. 17and a corresponding tension spring of belt tensioner815is not shown in the figures, is hooked into respective hollow inner walls of the tensioner holders816and826and the other ends of the tension springs are hooked into respective tension roller bearings. As the tension roller bearings are pulled and biased, the tension roller804applies tension to the cooling belt802while being supported to be rotatable.

In addition, on the outer faces of the front side plate810and the rear side plate820, tensioner holder supporting shafts810fand820fused for supporting the tensioner holders816and826to be swingable in directions Y1and Y2of arrows illustrated inFIGS. 16 and 17are integrally formed. The tensioner holders816and826can swing with respect to the tensioner holder supporting shafts810fand820fas the centers thereof. Accordingly, the tension roller804supported through the tension roller bearings818and828can reciprocate from a position contacting the cooling roller701illustrated inFIG. 13Ato a position separated from the cooling roller701illustrated inFIG. 13B.

In other words, among a plurality of suspension members for suspending the cooling belt802, the tension roller804located on the downstream in the conveying direction of the sheet is disposed to be movable so as to approach or be separated from the opposing cooling roller701. In addition, by moving the tension roller804, the nip length in the lowermost-stream portion in the contact range with the cooling belts802and702can be arbitrarily changed. This will be described more specifically.

As illustrated inFIGS. 14 and 15, near both end portions of a cam shaft900supported to be rotatable through bearings902and906by the front side plate810and the rear side plate820, on the outer side of the front side plate810and the rear side plate820, cams901and905having the same shape are fixed with the same phase. In addition, on the outer side of the cam905, a cam driving motor903using a pulse motor used for driving the cam shaft900to rotate is arranged on the outer side of the rear side plate820. Accordingly, the cams901and905can swing in the directions of arrows X1and X2illustrated inFIGS. 16 and 17in accordance with the rotation of the cam shaft900.

As illustrated inFIG. 15, above the cam905disposed on the outer face of the rear side plate820, a cam sensor904used for detecting an angular position of the cam905is arranged.

When the cam driving motor903starts to rotate in the direction (clockwise direction) of an arrow X2illustrated inFIG. 17, the cams901and905fixed to the cam shaft900rotate in the same direction. A detection flag portion905aformed integrally with one end of the cam905, as illustrated inFIG. 17, arrives between detection slits904aof the cam sensor904, it is detected that the cams901and905are located at an upper limit position illustrated inFIG. 16.

On the outer faces of the front side plate810and the rear side plate820, spring hook pins810eand820eare integrally formed. On one ends of the tensioner holders816and826, spring hook holes816aand826aare integrally formed. Pushing-up springs906and907of the pulling type can are hooked between the spring hook pins810eand820eand the spring hook holes816aand826a, and the tensioner holders816and826are biased so as to be pushed upwardly. Accordingly, the upper surfaces of the tensioner holders816and826contact the arc faces of the cams901and905, as illustrated inFIGS. 16 and 17, in accordance with biasing forces of the pushing-up springs906and907.

According to the configuration described above, the vertical positions of the tensioner holders816and826and the tension roller804supported to be rotatable by the tensioner holders816and826are controlled to be changed through the cams901and905by driving the cam driving motor903to rotate. The flow of control for lifting the tension roller804is illustrated inFIG. 18.

As illustrated inFIG. 13A, when the cam driving motor903starts to rotate from a position at which the tension roller804contacts the cooling roller701in the direction of arrow X2(clockwise direction) inFIG. 16in S181illustrated inFIG. 18, the cam faces of the cams901and905rise. In accordance therewith, the tensioner holders816and826rise in accordance with biasing forces of the pushing-up springs906and907. Accordingly, the tension roller804supported to be rotatable by the tensioner holders816and826rise in the direction of arrow Y2illustrated inFIG. 16with respect to the tensioner holder supporting shafts810fand820fas the centers thereof. Then, when the cam905is detected by the cam sensor904in S182, the cam driving motor903stops in S183. At this time, the tension roller804arrives at the upper limit position.

The state in which the tension roller804arrives at the upper limit position is illustrated inFIGS. 13B and 16. In addition, a block diagram of the sheet cooling apparatus according to this embodiment is illustrated inFIG. 19. As illustrated inFIG. 19, a CPU910as a controller controls the operation of the cam driving motor903through a motor controller911and a motor driver912in accordance with a signal transmitted from the cam sensor904.

As illustrated inFIG. 13B, the cooling belts702and802start contacting from the contact point A and passes the contact point B of the cooling rollers701and801and the contact point C between the belt pressure roller803and the cooling roller701, and thereafter, the cooling belts702and802are separated away from each other at a final contact point D′. At this time, the sheet P is discharged in the direction of arrow E′. A nip length from the contact point C contacting the belt pressure roller803to the final contact point D′ is N2.

For example, there is a case where a photograph image or the like for which the amount of toner T get onto the upper surface of the sheet P is much more than that of the case of a text image or the like for which the amount of toner is small is fixed to thin paper, particularly having low mass and low strength. In such a case, the adhesion between the toner T and the surface layer of the fixing roller110is firm inside the fixing nip portion N, and there is a case where an upward curl that is opposite to a downward curl occurs until the sheet is detached from the surface layer of the fixing roller110.

In addition, a cardboard or a coated sheet that receives more heat from the fixing device100than general plain paper having a weight of the sheet P of about 70 g to 80 g can easily maintain high temperature even after passing through the fixing device100. In addition, even when the weights of the sheets are equivalent, and the temperatures of sheets P after passing through the fixing device100are equivalent, a sheet having low rigidity has internal moisture that can be easily extracted to the outside, and a ripple tends to be remarkable therein.

As above, the contact range of the cooling belts702and802illustrated inFIG. 13B, that is, in the process of cooling the sheet P from the contact point A to the contact point D′, there is a case where the cooling performance for the sheet P is not sufficient, or the applying of a downward curl to the sheet P is not sufficient. In such a case, as below, the location of the tension roller804is lowered toward the cooling roller701. The flow of control for lowering the tension roller804is illustrated inFIG. 20.

When the cam driving motor903starts to rotate in a direction (the direction of arrow X1illustrated inFIG. 17) opposite to the direction of arrow X2inFIG. 16, in other words, in the counterclockwise direction in S201illustrated inFIG. 20, the cam faces of the cams901and905fall. In accordance therewith, the tensioner holders816and826and the tension roller804starts to fall in resistance against the biasing forces of the pushing-up springs906and907in the direction of arrow X1illustrated inFIG. 17with respect to the tensioner holder supporting shafts810fand820fas the centers thereof. Input pulses input to the cam driving motor903are started to be counted in S202, and, when the number of input pulses arrives at an arbitrary predetermined value in S203, the rotation of the cam driving motor903is stopped in S204. In a case where the predetermined value of the number of input pulses input to the cam driving motor903is set to a maximum value, as illustrated inFIG. 13A, the tension roller804contacts the cooling roller701through the cooling belts702and802.

At this time, as illustrated inFIG. 13A, the cooling belts702and802start contacting from the contact point A, and the cooling belts702and802are brought into surface contact with each other in a path (denoted by a thick line) up to the contact point D between the tension roller804and the cooling roller701. At this time, the sheet P is discharged in the direction of arrow E. A nip length from the contact point C with the belt pressure roller803to the final contact point D is N1. This nip length N1, as is apparent inFIGS. 13A and 13B, is longer than the above-described nip length N2up to the contact point D′.

As above, after the cooling belts702and802, as illustrated inFIGS. 13A and 13B, pass from the contact point A to the contact point B at which the cooling rollers701and801contact each other, the cooling belts702and802are brought into contact with each other in the entire area of the path (denoted by a thick line) over the contact point D or D′ for separation. Accordingly, when a sheet P is conveyed to the cooling apparatus, the sheet P is frictionally conveyed with the front surface and the rear surface being brought into surface contact with the cooling belts702and802along the circumferential faces of the cooling rollers801and701. In accordance with the surface contact, the heat contained in the sheet P is transferred to the cooling belts702and802and is radiated through the cooling rollers701and801.

Similarly to the first embodiment, the cooling rollers701and801have hollow inner portions, and a plurality of heat radiation fins is formed in a hollow inner wall in the shape of a spiral so as to have a surface area larger than that of the pipe shape. As a result, the heat radiation effect is high.

In addition, by causing cooling air to flow in the hollow inner spaces of the cooling rollers701and801in accordance with the cooling fans790and890, the heat radiation to the outside of the cooling apparatus102is promoted, which is the same as the first embodiment.

Furthermore, the advancement directions of the spirals of the heat radiation fins of the cooling rollers701and801, as illustrated inFIGS. 7A and 7B, are opposite to each other. Accordingly, although the rotation directions of the cooling rollers701and801are different from each other, the wind direction of cooling air generated by the heat radiation fins20coincides with the wind direction of the cooling air generated by the cooling fans790and890. As a result, the cooling air generated by the cooling fans790and890flow without blocking each other, which is the same as the first embodiment.

According to the configuration and the operation of the sheet cooling apparatus102described as above, in addition to the advantages of the above-described embodiments, the following advantages can be acquired. According to this embodiment, a nip length from the contact point C with the belt pressure roller803to the final the contact point D or D′ in the lowermost-stream portion of the contact range with the cooling belts702and802can be arbitrary selected (changed) in the range from an upper limit N1to a lower limit N2. Accordingly, the correction for the ripple and the curl that is optimal in accordance with the weight, the size, the paper type, the installation environment, the fixing temperature, and the like of the sheet P can be performed.

In the above-described embodiment, while the printer has been described as an image forming apparatus as an example, the present invention is not limited thereto. For example, the image forming apparatus may be another image forming apparatus such as a copying machine or a facsimile apparatus or another image forming apparatus such as a multi-function apparatus combining the functions thereof. In addition, the image forming apparatus is not limited to an image forming apparatus in which an intermediate transfer member is used, and toner images carried in the intermediate transfer member are transferred to a sheet together. Thus, the image forming apparatus may be an image forming apparatus in which a sheet bearing member is used, and toner images of colors are sequentially transferred to a sheet carried in the sheet bearing member in an overlapping manner. By applying the present invention to a sheet cooling apparatus of such an image forming apparatus, the same advantages can be acquired.

In addition, in the above-described embodiment, while the first and second cooling rollers, the first and second endless belts, and the first and second suspension members have been described to respectively have the same configuration and the same material, the present invention is not limited thereto. For example, in order to correct the curl of the sheet, it may be configured such that one cooling roller has a hollow inner portion and has a plurality of the heat radiation fins formed on the hollow inner wall in the shape of a spiral, and the other cooling roller is formed as a solid metal bar having a diameter smaller than the diameter of the one cooling roller.

This application claims the benefit of Japanese Patent Application No. 2012-166069, filed Jul. 26, 2012, which is hereby incorporated by reference herein in its entirety.