Fixing device and image forming apparatus

A fixing device includes a fixing rotator, a pressing rotator, and a guide. The pressing rotator forms a fixing nip with the fixing rotator. The guide guides a recording medium to the fixing nip. The guide includes a guide surface that contacts a surface of the recording medium against a guide the recording medium. The guide surface is provided parallel to a line tangent to the fixing rotator and the pressing rotator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is based on and claims priority pursuant to 35 U.S.C. § 119 to Japanese Patent Application No. 2015-127798 filed on Jun. 25, 2015, and Japanese Patent Application No. 2016-024644 filed on Feb. 12, 2016 in the Japan Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein.

BACKGROUND

Technical Field

Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multi-function peripheral including the fixing device, the multi-function peripheral having at least two of the copying, printing, and facsimile functions.

Related Art

Image forming apparatuses such as copiers, printers, facsimile machines, or multi-function peripherals having at least two of copying, printing, and facsimile functions include a fixing device that fixes a developer image on a recording medium. The fixing device includes a fixing rotator and a pressing rotator to form a fixing nip through which the recording medium is passed, so that the developer image on the recording medium is fixed with heat and pressure in the fixing nip. Such a fixing device includes a guide for correctly guiding the recording medium to the fixing nip.

For example, as illustrated inFIG. 9, a fixing device includes an inlet guide102, a heating roller100, and a pressing roller101. The heating roller100and the pressing roller101form a fixing nip N9. The inlet guide102is disposed on an upstream side of the fixing nip N9in a sheet conveyance direction. A sheet P is guided and conveyed to the fixing nip N9by the inlet guide102while contacting a leading end contact portion102aof the inlet guide102. Thus, the sheet P is conveyed in an S-curved state, and the leading end contact portion102acancels out a conveyance speed difference between a front surface and a back surface of the sheet P.

SUMMARY

In at least one embodiment of this disclosure, there is provided an improved fixing device that includes a fixing rotator, a pressing rotator, and a guide. The pressing rotator forms a fixing nip with the fixing rotator. The guide guides a recording medium to the fixing nip. The guide includes a guide surface that contacts a surface of the recording medium against a guide the recording medium. The guide surface is provided parallel to a line tangent to the fixing rotator and the pressing rotator.

DETAILED DESCRIPTION

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

InFIG. 1, a color image forming apparatus1includes an image forming unit2in a middle portion thereof. The image forming unit2includes four process units9Y,9M,9C, and9K, each of which is detachably disposed. The process units9Y,9M,9C, and9K respectively store yellow (Y), magenta (M), cyan (C), and black (K) developers of different colors for color separation components of a color image. Each of the process units9Y,9M,9C, and9K is substantially similar to every other except for the color of developer therein.

In particular, the process units9Y,9M,9C, and9K respectively include photoconductor drums10Y,10M,10C, and10K, charging rollers11Y,11M,11C, and11K, and developing devices12Y,12M,12C, and12K. Each of the photoconductor drums10Y,10M,10C, and10K as drum-shaped rotators includes a surface that can bear toner serving as developer. The charging rollers11Y,11M,11C, and11K uniformly charge the surfaces of the respective photoconductor drums10Y,10M,10C, and10K. The developing devices12Y,12M,12C, and12K respectively include developing rollers13Y,13M,13C, and13K that supply toner to the surfaces of the respective photoconductor drums10Y,10M,10C, and10K.

Moreover, in an upper portion of the image forming apparatus1, toner bottles26Y,26M,26C, and26K respectively filled with yellow, cyan, magenta, and black toners are detachably arranged. The yellow, cyan, magenta, and black toners stored in the toner bottles26Y,26M,26C, and26K are respectively supplied to the developing devices12Y,12M,12C, and12K via supply paths arranged between the toner bottles26Y,26M,26C, and26K and the respective developing devices12Y,12M,12C, and12K.

Moreover, the image forming apparatus1includes an exposure unit3disposed below the process units9Y,9M,9C, and9K. The exposure unit3emits a laser beam based on image data.

The image forming apparatus1includes a transfer unit4disposed above the image forming unit2. The transfer unit4includes an endless intermediate transfer belt16, primary transfer rollers17Y,17M,17C, and17K, a secondary transfer roller18as a transfer rotator, a secondary transfer backup roller14as an opposite rotator, a cleaning backup roller15, a tension roller27, and a belt cleaning device28. The primary transfer rollers17Y,17M,17C, and17K are arranged in positions opposite the respective photoconductor drums10Y,10M,10C, and10K of the process units9Y,9M,9C, and9K with the intermediate transfer belt16therebetween.

The intermediate transfer belt16is an endless belt and is entrained around the secondary transfer backup roller14, the cleaning backup roller15, and the tension roller27. Herein, rotation of the secondary transfer backup roller14moves (rotates) the intermediate transfer belt16in a direction indicated by an arrow A shown inFIG. 1.

The four primary transfer rollers17Y,17M,17C, and17K and the respective photoconductor drums10Y,10M,10C, and10K nip the intermediate transfer belt16to form primary transfer nips. Each of the primary transfer rollers17Y,17M,17C, and17K is connected to a power source so as to receive a predetermined direct current (DC) voltage and/or a predetermined alternating current (AC) voltage.

The secondary transfer roller18and the secondary transfer backup roller14nip the intermediate transfer belt16to form a secondary transfer nip M as a transfer nip. Similar to the primary transfer rollers17Y,17M,17C, and17K, the secondary transfer roller18is connected to a power source so as to receive a predetermined DC voltage and/or a predetermined AC voltage.

The belt cleaning device28includes a cleaning brush and a cleaning blade that are arranged to contact the intermediate transfer belt16. A waste toner collected by the belt cleaning device28is stored in a waster toner bottle via a waster toner transporting hose.

A sheet feeding unit5is disposed in a lower portion of the image forming apparatus1. The sheet feeding unit5includes a sheet cassette19for storing sheets P as recording media, and a sheet feeding roller20for conveying a sheet P from the sheet cassette19.

A sheet P fed from the sheet feeding unit5is conveyed along a conveyance path6. A plurality of conveyance roller pairs including a registration roller pair21is arranged as appropriate along the conveyance path6to a sheet ejection unit8.

A fixing device7includes a fixing roller22as a fixing rotator, and a pressing roller23as a pressing rotator. The fixing roller22is heated by a heat source, and the pressing roller23can press the fixing roller22. The pressing roller23and the fixing roller22form a fixing nip N.

The sheet ejection unit8is disposed on the extreme downstream side of the conveyance path6in the image forming apparatus1. The sheet ejection unit8includes a pair of ejection rollers24for ejecting the sheet P outside, and an ejection tray25on which the ejected sheet P is stacked.

The image forming unit2, the exposure unit3, and the transfer unit4function to form an image on the sheet P.

Hereinafter, a basic operation of the image forming apparatus1is described with reference toFIG. 1.

When the image forming apparatus1starts an image forming operation, electrostatic latent images are formed on surfaces of the photoconductor drums10Y,10M,10C, and10K of the respective process units9Y,9M,9C, and9K. Herein, the exposure unit3emits beams to the photoconductor drums10Y,10M,10C, and10K based on image information that is single color image information obtained by dividing a desired full-color image into color information of yellow, cyan, magenta, and black. When the electrostatic latent images are formed on the photoconductor drums10Y,10M,10C, and10K, the drum-shaped developing rollers13Y,13M,13C, and13K respectively supply the toners stored in the developing devices12Y,12M,12C, and12K to the photoconductor drums10Y,10M,10C, and10K. Thus, the electrostatic latent images become visible as toner images (developer images).

In the transfer unit4, when the secondary transfer backup roller14is rotated, the intermediate transfer belt16moves in a direction indicated by the arrow A shown inFIG. 1. Moreover, a constant voltage or a voltage with current that is maintained constant is applied to each of the primary transfer rollers17Y,17M,17C, and17K, the constant voltage or the constant current voltage having a polarity opposite to a toner charge polarity. This forms transfer electric fields in the primary transfer nips, so that the toner images on the photoconductor drums10Y,10M,10C, and10K are sequentially superimposed and transferred to the intermediate transfer belt16in the primary transfer nips.

Meanwhile, in the lower portion of the image forming apparatus1, the sheet feeding roller20of the sheet feeding unit5is rotated to feed a sheet P stored in the sheet cassette19to the conveyance path6when the image forming apparatus1starts the image forming operation. The registration roller pair21times conveyance of the sheet P fed to the conveyance path6, so that the sheet P is conveyed to the secondary transfer nip M between the secondary transfer roller18and the secondary transfer backup roller14. Herein, a transfer voltage having a polarity opposite to the toner charge polarity of the toner image on the intermediate transfer belt16is being applied, and a transfer electric field is being formed in the secondary transfer nip M. With the electric field in the secondary transfer nip, the toner images on the intermediate transfer belt16are collectively transferred to the sheet P.

The sheet P with the transferred toner image is conveyed to the fixing device7, so that the toner image is fixed on the sheet P with heat and pressure applied by the fixing roller22and the pressing roller23. Subsequently, the sheet P with the fixed image is separated from the fixing roller22and then conveyed by the conveyance roller pair to the sheet ejection unit8in which the sheet P is ejected by the ejection roller pair24to the ejection tray25.

The image forming operation has been described using an example case in which a full-color image is formed on a sheet P. However, the image forming operation can be applied to a case in which a single color image is formed using any one of the four process units9Y,9M,9C, and9K, or a two-color image or a three-color image is formed using two or three out of the four process units9Y,9M,9C, and9K.

On the conveyance path6, a guide30for guiding the sheet P to the fixing nip N of the fixing device7is disposed on an upstream side of the fixing device7in a conveyance direction of the sheet P (hereinafter, the conveyance direction of the sheet P is also called a conveyance direction).

FIG. 2is a diagram of a related-art fixing device including a fixing roller220, a pressure roller230, and a guide40. InFIG. 2, the guide40, made of a resin material, is disposed on an upstream side of a fixing nip NN including a fixing nip surface NN1in a conveyance direction. The guide40includes an edge portion401that contacts a sheet P at a point to guide the sheet P to the fixing nip NN.

In such a configuration, however, an electric charge tends to be accumulated in a position of the edge portion401. When a surface of the sheet P slides against the edge portion401, the edge portion401is charged. This causes a problem of irregularity in a toner image on the surface of the sheet P.

The guide preferably has an electrical resistance of approximately 108Ω to approximately 1012Ω. In a case in which the guide is made of a conductive material, an electric charge retained by the sheet P or an electric charge applied to the sheet P by a transfer unit flows to the guide when the sheet P contacts the guide. This causes the irregularity in the toner image on the sheep P and a transfer process failure in the transfer unit. Although such failures can be prevented if a coating agent or a paste having moderate electrical resistance is applied to a surface of the guide, the cost of the guide increases as a result.

Moreover, in a case in which a guide is made of a non-conductive material similar to the related-art guide40illustrated inFIG. 2, the guide is charged when a sheet P slides against the guide. This causes an irregularity in a toner image on the sheet P. Such a failure can be resolved if electrical resistance is reduced by adding a material such as carbon to the guide. However, such a method causes an increase in cost and degradation in moldability.

The guide30of the present exemplary embodiment is configured to solve such problems. Hereinafter, the guide30is described.

According to the present exemplary embodiment, as illustrated inFIG. 3, a portion of the guide30on a downstream side in the conveyance direction is chamfered toward the fixing roller22(the chamfered portion is indicated by a dotted line inFIG. 3). The guide30includes a first guide surface301and a second guide surface302each serving as a guide surface for guiding a sheet P to the fixing nip N. The first guide surface301and the second guide surface302are continuous with each other. A boundary304as a border between the first guide surface301and the second guide surface302has a curved surface with a radius of 0.2 mm to 0.3 mm. Since the boundary304has the curved surface, a friction with the sheet P in the boundary304can be reduced. The first guide surface301includes an end portion303as a downstream end portion on the downstream side in a conveyance direction.

Next, relative positions of the guide30and each components of the fixing device7according to the present exemplary embodiment of the present invention is described with reference toFIG. 4.

In the present exemplary embodiment, as illustrated inFIG. 4, the first guide surface301is disposed parallel to a line tangent N2to the fixing roller22and the pressing roller23(or a fixing nip surface N1of the fixing nip N). Accordingly, as illustrated inFIG. 5, when a sheet P is conveyed to the fixing nip N, such arrangement enables the first guide surface301to guide the sheet P to the fixing nip N while a surface of the first guide surface301contacts the sheet P as a plane without projections with respect to the sheet P.

The first guide surface301is provided on the extreme downstream side of the guide30in the conveyance direction. Moreover, as illustrated inFIG. 4, the end portion303of the first guide surface301and an inlet portion of the fixing nip N are spaced apart by approximately 7 mm to approximately 10 mm.

As illustrated inFIG. 4, the second guide surface302is an inclined surface that is inclined from the side of the pressing roller23toward the side of the fixing roller22from the upstream side to the downstream side in the conveyance direction. Such an inclined surface of the second guide surface302enables the sheet P to be guided to a direction indicated by an arrow C shown inFIG. 4and to the side of the fixing roller22. Since the second guide surface302has a major function of guiding a leading end of the sheet P, the second guide surface302preferably has smoothness with a low friction coefficient.

Moreover, the entire area of the first guide surface301(in particular, the end portion303of the first guide surface301) is arranged on the side of the fixing roller22relative to the line tangent N2.

Such arrangement facilitates conveyance of the sheet P along a surface of the fixing roller22when the sheet P is guided from the first guide surface301to the fixing nip N. Since the fixing roller22has higher rigidity than the pressing roller23, the conveyance of the sheet P along the surface of the fixing roller22enables not only a conveyance position of the sheet P to be stable but also the sheet P to be conveyed to the fixing nip N with higher accuracy.

Although the arrangement of the first guide surface301on the side of the fixing roller22can provide the above effects, a friction between the first guide surface301and the sheet P increases. This increases a charge on the first guide surface301. That is, since the sheet P is nipped between the first guide surface301and the surface of fixing roller22of the fixing nip N, the friction between the sheet P and the first guide surface301is high in comparison with a case in which the first guide surface301is arranged on the line tangent N2or the side closer to the pressing roller23.

Even in such a case, the arrangement of the first guide surface301parallel to the line tangent N2in the guide30of the present exemplary embodiment can reduce the charge on the first guide surface301. Accordingly, even if a friction between the sheet P and the first guide surface301is increased, the arrangement of the first guide surface301on the side of the fixing roller22can prevent the charge on the first guide surface301from being excessively large. Hence, a toner image on the surface of the sheet P does not tend to be affected.

Since the guide30is disposed near an inlet of the fixing nip N, temperature of the guide30is liable to increase. Hence, the guide30needs to have resistance to heat. Moreover, a user may touch the guide30when dealing with a paper jam. Thus, the guide30needs to be made of a material that can keep a surface temperature to 80 degrees Celsius or below. Moreover, the guide30needs to have surface smoothness so that a friction resistance is not increased when the guide30guides the sheet P.

Therefore, the guide30of the present exemplary embodiment is made of a heat-resistant resin material such as liquid crystal polymer (LCP) that is capable of forming a smooth surface and has good size stability. The resin material can be mixed with a reinforcing filler to maintain heat resistance and strength.

However, there is a possibility that the mixture may be deposited on the surface of the guide30. In some cases, such deposition of the mixture can affect the surface smoothness of the guide30. Consequently, the mixture is not used in the present exemplary embodiment. Moreover, an amount of a material such as carbon to be blended is set to be low from a molding standpoint.

If a surface roughness of the first guide surface301is excessively low, a contact area between the first guide surface301and the sheet P increases. This may cause an increase in a charge on the guide30. On the other hand, if a surface roughness of the first guide surface301excessively high, the rough surface may interrupt guidance of the sheet P.

Herein, as for the first guide surface301of the present exemplary embodiment, a surface roughness can be determined in consideration of charge protection. The surface roughness of the first guide surface301is set to approximately 50 μm to approximately 150 μm.

The surface roughness of the first guide surface301can be set by a crimping process that is performed on the surface of the first guide surface301. However, as long as the above surface roughness can be acquired, other processes can be employed.

Hereinafter, a fixing device of another exemplary embodiment of the present exemplary embodiment is described with reference toFIG. 6. The fixing apparatus of the present exemplary embodiment is applied to a monochrome image forming apparatus1000illustrated inFIG. 6.

First, a description is given mainly of differences between the image forming apparatus1illustrated inFIG. 1and the image forming apparatus1000illustrated inFIG. 6.

The image forming apparatus1000of the present exemplary embodiment includes a sheet feeding unit5, an image forming unit2, a fixing device7, and an sheet ejection unit8.

The image forming unit2includes an exposure device, a developing device, a cleaning unit. In the image forming unit2, a toner stored in the developing device is supplied to a photoconductor drum10as an opposite rotator, so that an electrostatic latent image on the photoconductor drum10becomes visible as a toner image.

Meanwhile, a registration roller21times conveyance of a sheet P fed from the sheet feeding unit5to convey the sheet P to a nip portion at a transfer roller34as a transfer rotator. In the nip portion, the toner image is transferred to the sheet P.

The sheet P with the transferred image is conveyed to the fixing device7via a conveyance path, so that the toner image is fixed on the sheet P with heat and pressure applied by a fixing roller22and a pressing roller23.

The sheet P with the fixed image is separated from the fixing roller22by a separation unit35, and then further conveyed to a downstream side.

Subsequently, in a sheet ejection unit8, the sheet P is ejected by an ejection roller pair24to an ejection tray25.

FIG. 7is a diagram illustrating a configuration of the fixing device7and the periphery thereof according to the present exemplary embodiment, andFIG. 8is a detailed diagram of the configuration illustrated inFIG. 7and includes a relative position of each components of the fixing device7. In the fixing device7as illustrated inFIGS. 7 and 8, a first guide surface301of a guide30is provided parallel to a common line tangent D to the fixing roller22and the photoconductor drum10(in an example illustrated inFIGS. 7 and 8, one portion of the line tangent D matches the first guide surface301). The line tangent D to the fixing roller22and the photoconductor drum10is approximated to a conveyance path6(seeFIG. 1) on which the sheet P is conveyed from a transfer nip MM to a fixing nip N. Thus, the parallel arrangement of the first guide surface301to the line tangent D allows the first guide surface301to be substantially parallel to the sheet P being conveyed and to contact the sheet P as a plane. Therefore, an electric charge does not tend to be accumulated on the first guide surface301when the first guide surface301guides the sheet P. This can reduce adverse effects on the toner image. The adverse effects include a case in which an irregularity of the toner image on the surface of the sheet P due to the guide30causes image dust particles.

On an upstream side of the fixing nip N in the conveyance direction, an opposite guide33is disposed opposite the guide30. The opposite guide33is inclined toward the fixing nip N (or the line tangent D) as it approaches the downstream side from the upstream side in the conveyance direction. This enables the conveyance path along which the sheet P is conveyed to be narrowed toward the fixing nip N, thereby guiding the sheet P in a correct conveyance direction.

The guide30can be set as a first guide. In such a case, on an upstream side of the first guide30in the conveyance direction, a second guide31and a third guide32for guiding a sheet P are arranged on the side of the pressing roller23relative to the line tangent D (the side opposite the first guide surface301with the line tangent D therebetween). Moreover, a space B is provided between the line tangent D and the first guide30, the second guide31, the third guide32, and a transfer roller34. The space B serves as a predetermined area in which the sheet P can pass.

A rotation speed of each of the fixing roller22and the pressing roller23is set to be lower than that of each of the transfer roller34and the photoconductor drum10. A linear velocity in the fixing nip N is lower than that in the transfer nip MM, so that the sheet P is prevented from being pulled toward the fixing device7. Herein, a linear velocity difference between the fixing nip N and the transfer nip MM causes slack of the sheet P. However, the sheet P can be slackened off to absorb the slack in the space B.

In a case in which the second guide31and the third guide32are arranged on the line tangent D or the side closer to the fixing roller22, the sheet P slides against the second guide31or the third guide32with a large friction force when being conveyed from the transfer nip MM to the fixing nip N. This charges the second guide31and the third guide32, causing a failure such as the aforementioned image dust particles.

In the present exemplary embodiment, the first guide30, the second guide31, and the third guide32are arranged on the side of the pressing roller23relative to the line tangent D. Such arrangement enables the sheet P to be slackened toward the pressing roller23in the space B, thereby preventing a large friction force from being generated due to strong slide of the sheet P against the second guide31and the third guide32. The second guide31and the third guide32can be provided as one unit, or three or more guides can be arranged.

In the present exemplary embodiment, the first guide surface301can be arranged on the side of the fixing roller22relative to the line tangent D.

According to the fixing device of the exemplary embodiment of the present invention, therefore, a guide surface for guiding a recording medium is provided parallel to a line tangent to a fixing rotator and a pressing rotator. This construction enables the recording medium to contact the guide surface as a plane without any projection of the guide surface with respect to the recording medium when the recording medium is conveyed from the guide surface to a fixing nip. Accordingly, a charge on the guide surface when the recording medium is guided can be low in comparison with a case in which a recording medium contacts a projection of the guide surface as a point. Hence, adverse effects such as image irregularities that occur when a developer image on the recording medium surface is fixed can be reduced.

Therefore, at least one exemplary embodiment of the present invention can provide a fixing device including a low-cost guide that can guide a recording medium to a fixing nip and reduce a charge amount when a recoding medium is guided.

Embodiments of the present invention have been described above with reference to specific exemplary embodiments. Note that the present disclosure is not limited to the details of the embodiments described above, but various modifications and enhancements are possible without departing from the scope thereof.

In the exemplary embodiment described above with reference toFIG. 6, an image forming apparatus employing a direct transfer method is used as an example. However, such an exemplary embodiment can be applied to an image forming apparatus employing an intermediate transfer method as illustrated inFIG. 1. Moreover, a configuration of the image forming apparatus described above with reference toFIG. 1can be applied to the image forming apparatus employing the direct transfer method illustrated inFIG. 6.