Image forming apparatus having a rockable induction heating unit

An image forming apparatus includes an apparatus main body, a fixing unit, an induction heating unit, a positioning mechanism, and a charge adjustment portion. The fixing unit includes a first fixing member and a second fixing member that form, between them, a nip portion through which a sheet is passed. The induction heating unit is supported so as to be rockable with respect to the apparatus main body and heats the first fixing member. The positioning mechanism keeps a distance between the induction heating unit and the first fixing member constant. The charge adjustment portion is fixed to the induction heating unit in a detachable manner and electrically charges or eliminates static electricity from the first fixing member.

INCORPORATION BY REFERENCE

This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2016-085051 filed on Apr. 21, 2016, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present disclosure relates to an electrophotographic image forming apparatus.

In electrophotographic image forming apparatuses, a sheet with a toner image formed thereon is passed through a nip portion between a pair of fixing members which each may be a roller or a belt, and the sheet is heated and pressed while being passed through the nip portion so that the toner image is fixed to the sheet.

In such image forming apparatuses, an offset phenomenon may occur, wherein in the offset phenomenon, a part of the toner transfers to the surface of the fixing members. There is known a fixing device in which, to prevent the offset phenomenon, a discharging brush that changes the charge amount on a surface of a heating roll, is provided at a position that faces the surface of the heating roll, and the voltage of the discharging brush is controlled based on the density of the toner image.

SUMMARY

An image forming apparatus according to an aspect of the present disclosure includes an apparatus main body, a fixing unit, an induction heating unit, a positioning mechanism, and a charge adjustment portion. The fixing unit includes a first fixing member and a second fixing member that form therebetween a nip portion through which a sheet is passed. The induction heating unit is supported so as to be rockable with respect to the apparatus main body and heats the first fixing member. The positioning mechanism keeps a distance between the induction heating unit and the first fixing member constant. The charge adjustment portion is fixed to the induction heating unit in a detachable manner and electrically charges or eliminates static electricity from the first fixing member.

DETAILED DESCRIPTION

The following describes an embodiment of the present disclusure with reference to the accompanying drawings for the understanding of the present disclosure. It should be noted that the following embodiment is an example of a specific embodiment of the present disclosure and should not limit the technical scope of the present disclosure.

[Configuration of Image Forming Apparatus]

First, a description is given of a configuration of an image forming apparatus1according to an embodiment of the present disclosure, with reference toFIG. 1. As shown inFIG. 1, the image forming apparatus1includes a sheet cassette2, an image generating portion3, a fixing portion4, and a control portion5. It is noted that the present disclosure is not limited to a printer, but is applicable to an arbitrary image forming apparatus such as a copier, a facsimile device, and a multifunction peripheral.

The sheet cassette2stores sheets such as sheets of recording paper.

The image generating portion3includes a photoconductor drum, a charger, an exposure device, a developing device, and a transfer device. The image generating portion3forms a toner image on a sheet fed from the sheet cassette2.

The fixing portion4heats and presses a sheet with a toner image formed thereon such that the toner image is fixed to the sheet. A concrete configuration of the fixing portion4for fixing the toner image to the sheet is described below.

The control portion5includes control equipment such as CPU, ROM, and RAM. The CPU is a processor that executes various calculation processes. The ROM is a nonvolatile storage portion in which various information such as control programs for causing the CPU to execute various processes are stored in advance. The RAM is a volatile or nonvolatile storage portion that is used as a temporary storage memory (working area) for the various processes executed by the CPU. It is noted that the control portion5may include an electronic circuit that realizes the various processes.

The fixing portion4includes a fixing unit10, an induction heating unit20, and a charge application portion30(an example of the charge adjustment portion and the contactless charger of the present disclosure).

The fixing unit10includes a fixing belt11(an example of the first fixing member of the present disclosure) and a pressure roller12(an example of the second fixing member of the present disclosure), wherein the fixing belt11and the pressure roller12form therebetween a nip portion16through which a sheet S passes (seeFIG. 3). The fixing unit10is attached to an apparatus main body6of the image forming apparatus1in a detachable manner. Specifically, the apparatus main body6of the image forming apparatus1includes a main body cover7that can be opened and closed, and as shown inFIG. 2, the fixing unit10is attached to and detached from the apparatus main body6in a state where the main body cover7is opened.

The induction heating unit20is supported so as to be rockable with respect to the apparatus main body6and configured to heat the fixing belt11. It is noted that in a state where the fixing unit10is attached to the apparatus main body6, a distance between the induction heating unit20and the fixing belt11is kept to be constant by a positioning mechanism40(seeFIG. 7) that is described below. Accordingly, for example, even after the fixing unit10is replaced with a new one, the distance between the induction heating unit20and the fixing belt11is kept to be constant and it is not necessary to finely adjust the distance between the induction heating unit20and the fixing belt11. Accordingly, the induction heating unit20can uniformly heat the fixing belt11.

The charge application portion30is fixed to the induction heating unit20in a detachable manner. The charge application portion30charges the fixing belt11by using the corona discharge.

Next, a more detailed description is given of the configuration of the fixing portion4with reference toFIG. 3.

As shown inFIG. 3, the fixing unit10includes the fixing belt11, the pressure roller12, a holding member13, a nip forming member14, a guide plate15, a conveyance guide17, and a separation plate18, wherein the holding member13, the nip forming member14, and the guide plate15are disposed inside the fixing belt11.

The fixing belt11is formed in a substantially cylindrical shape that is elongated in a width direction perpendicular to the conveyance direction of the sheet S (hereinafter, the direction is merely referred to as a “width direction”). The fixing belt11is supported by the holding member13, the nip forming member14, and the guide plate15in such a way as to rotate around a rotation axis that extends in the width direction. The holding member13, the nip forming member14, and the guide plate15are an example of the holding member of the present disclosure.

The fixing belt11is composed of a base layer, an elastic layer and a release layer, wherein the elastic layer is formed on the base layer, and the release layer is formed to cover the elastic layer. The base layer is, for example, formed by the nickel electrocasting or by performing the plating treatment or the rolling treatment on a metal such as copper. The elastic layer is formed from silicon rubber, for example. The release layer is formed from fluororesin such as PFA, for example.

The pressure roller12is formed in a substantially cylindrical shape that is elongated in the width direction. The pressure roller12is pressed against the fixing belt11by a pressure mechanism (not shown), and the nip portion16is formed between the fixing belt11and the pressure roller12. The pressure roller12is rotatably supported by a fixing frame (not shown). The pressure roller12is rotationally driven by a driving mechanism (not shown).

The pressure roller12is composed of, for example, a cylindrical core material, an elastic layer and a release layer, wherein the elastic layer is formed on the core material, and the release layer is formed to cover the elastic layer. The core material is formed, for example, from a metal such as stainless steel or aluminum. The elastic layer is formed, for example, from silicon rubber or silicon sponge. The release layer is formed, for example, from fluororesin such as PFA.

The induction heating unit20includes induction coils21, coil holding portions22, arch cores23a, a center core23b, side cores23c, arch core holding portions24, and a cover portion25, wherein the induction coils21are disposed in an arc shape along an outer circumference of the fixing belt11, the coil holding portions22hold the induction coils21, and the arch core holding portions24hold the arch cores23a.

When toner T (seeFIG. 5) is fixed to the sheet S, a high-frequency current is applied to the induction coils21. This causes the induction coils21to generate a magnetic field. An eddy current is then generated in the fixing belt11by the act of the magnetic field, and the fixing belt11is heated. That is, the fixing belt11is heated by the induction coils21. In addition, the guide plate15is heated by the act of the magnetic field, and the fixing belt11is heated also by the guide plate15.

In addition, when the toner T is fixed to the sheet S, the pressure roller12is rotationally driven by a driving mechanism (not shown). This causes the fixing belt11that is pressed against the pressure roller12, to be rotated following the rotation of the pressure roller12. The fixing belt11rotated in this way slides over the nip forming member14. In this situation, when the sheet S enters the nip portion16, the heated fixing belt11contacts the unfixed toner T on the sheet S. This causes the toner T to be fused, pressed, and fixed to the sheet S. After it passes through the nip portion16, the sheet S is separated from the fixing belt11by the separation plate18, and is discharged to outside the fixing unit10.

Meanwhile, in the fixing portion4having the above-described configuration, an offset phenomenon may occur in which a part of the toner T transfers to the surface of the fixing belt11. It is considered that, to prevent the offset phenomenon, a charge adjustment portion such as a discharging brush that changes the charge amount on the surface of the fixing belt11, may be provided at a position that faces the surface of the fixing belt11. In this case, unevenness of the charge amount on the surface of the fixing belt11may occur if the distance between the surface of the fixing belt11and the charge adjustment portion is not uniform along the axial direction. According to the present embodiment, the charge application portion30is fixed to the induction heating unit20, and thus the charge application portion30can be set easily at an appropriate position.

In the present embodiment, it is supposed that unfixed toner T on the sheet S is positively charged. On the other hand, fluororesin such as PFA used in the release layer of the fixing belt11is likely to be negatively charged. Thus in this state, the offset phenomenon is likely to occur in which the positively charged toner T transfers to the negatively charged surface of the fixing belt11. In view of this, in the present embodiment, the charge application portion30applies an electric charge of the same polarity as that of the toner T on the sheet S (namely, the positive polarity) to the surface of the fixing belt11. With this configuration, since the surface of the fixing belt11is charged to the same polarity as that of the toner T, the transfer of the toner T to the surface of the fixing belt11is effectively restricted.

[Configuration of Charge Application Portion]

The charge application portion30includes a discharge electrode31, a pair of counter electrodes32and33, an electrode holding portion34, and a bolt35. The counter electrodes32and33face each other across the discharge electrode31. The electrode holding portion34holds these electrodes. The bolt35fixes the electrode holding portion34to the coil holding portions22of the induction heating unit20in a dechable manner.

The discharge electrode31and the counter electrodes32and33are disposed apart from the outer circumferential surface of the fixing belt11. As shown inFIG. 3, the discharge electrode31is a plate-like electrode made of stainless steel that is thin in plate thickness (approximately 0.1 mm), wherein saw teeth are formed along the width direction on its edge portion facing the fixing belt11. The counter electrodes32and33are plate-like electrodes made of stainless steel, and are disposed apart by a predetermined distance from pointed end portions31aof the teeth formed on the discharge electrode31.

A high voltage power source (not shown) is connected to the discharge electrode31, and the counter electrodes32and33are grounded. When the fixing portion4performs a fixing operation, a high voltage is applied to the discharge electrode31under the control of the control portion5. When the high voltage is applied to the discharge electrode31, a corona discharge is continuously generated between the pointed end portions31aof the teeth formed on the discharge electrode31and the counter electrodes32and33. Positive ions generated by the corona discharge move away from the pointed end portions31aof the discharge electrode31, and the surface of the fixing belt11is positively charged by a part of those positive ions. As a result, as shown inFIG. 5, the surface of the fixing belt11that had been negatively charged is positively charged by the positive ions coming from the charge application portion30, and while keeping that state, the surface of the fixing belt11moves toward the nip portion16. This allows the toner T on the sheet S and the surface of the fixing belt11to have the same polarity in the nip portion16, which makes it possible to restrict the transfer of the toner T to the surface of the fixing belt11.

Next, a description is given of the positioning mechanism40that keeps the distance between the induction heating unit20and the fixing belt11constant, with reference toFIG. 6andFIG. 7.

FIG. 6is a diagram showing the induction heating unit20and the fixing belt11viewed from above.FIG. 7is a diagram showing the induction heating unit20and the fixing belt11viewed from a side along the width direction.

In the present embodiment, first positioning members26, second positioning members19, and a compression spring41are provided as the positioning mechanism40. The first positioning member26are respectively fixed to opposite ends of the induction heating unit20in the width direction. The second positioning members19are respectively fixed to opposite ends of the holding member13in the width direction, the holding member13holding the fixing belt11. One end of the compression spring41is fixed to the apparatus main body6, and the other is fixed to the induction heating unit20.

The induction heating unit20is rockably supported by the apparatus main body6. Specifically, as shown inFIG. 7, the induction heating unit20is supported so as to be pivotable around a support point27with respect to the apparatus main body6, and is biased toward the fixing unit10by the compression spring41.

The second positioning members19has an approximately cylindrical shape, and is configured such that its central axis matches the rotation axis of the fixing belt11.

The induction heating unit20is biased toward the fixing unit10by the compression spring41. Thus, when the fixing unit10is attached to a predetermined position of the apparatus main body6, as shown inFIG. 7, the first positioning members26and the second positioning members19are pressed against each other by the compression spring41, and the positional relationship between the first positioning members26and the second positioning members19is fixed. As a result, a distance between the fixing belt11and the induction coils21of the induction heating unit20becomes uniform along the width direction. This makes it possible for the induction coils21to heat the fixing belt11uniformly.

In the present embodiment, the charge application portion30is fixed to the coil holding portions22of the induction heating unit20. Since the positional relationship between the first positioning members26and the coil holding portions22is fixed, the distance between the fixing belt11and the charge application portion30is also made uniform along the width direction by the positioning mechanism40. In particular, when a contactless charger such as the charge application portion30is used, originally, it is not easy to keep the distance of it from the fixing belt11constant along the width direction. However, according to the present embodiment, even a contactless charger such as the charge application portion30is used, it is possible to easily keep the distance of it from the fixing belt11constant along the width direction.

It is noted that the image forming apparatus1includes an adjustment mechanism configured to adjust an inclination of the fixing unit10so that the conveyance direction of the sheet S by the nip portion16can be adjusted appropriately. For example, the inclination of the fixing unit10may be adjusted by changing the position (height) of the guide member that is provided in the apparatus main body6to support the fixing unit10. In the present embodiment, the charge application portion30is attached to the induction heating unit20. Accordingly, even if the inclination of the fixing unit10is changed by the adjustment mechanism, the distance between the fixing belt11and the induction heating unit20is kept constant by the positioning mechanism40, and thus the distance between the fixing belt11and the charge application portion30is also kept constant. As a result, according to the present embodiment, it is possible to easily set the charge application portion30at an appropriate position.

In addition, the charge application portion30easily becomes dirty due to scattered paper dust and toner, and thus is replaced with a new one as necessary. According to the present embodiment, as shown inFIG. 2, the attachment position of the charge application portion30to the induction heating unit20is exposed to outside in a state where the fixing unit10is detached from the apparatus main body6. Thus, according to the present embodiment, the charge application portion30can be easily replaced.

In addition, in the present embodiment, the attachment position of the charge application portion30to the induction heating unit20is a side surface22aof the induction heating unit20that is located on the downstream side in the moving direction of the outer circumferential surface of the fixing belt11disposed to face the induction heating unit20(seeFIG. 3andFIG. 7). As a result, as shown inFIG. 5, the surface of the fixing belt11is positively charged immediately before it reaches the nip portion16. Accordingly, the surface of the fixing belt11reaches the nip portion16while it is in the positively charged state in a more reliable manner than a case where the charge application portion30is attached to a side surface22bthat is on an upper portion of the induction heating unit20as shown inFIG. 3.

It is noted that according to the present embodiment, it is supposed that the toner T on the sheet S is positively charged. However, in a case where the toner T on the sheet S is negatively charged, a negative voltage may be applied to the discharge electrode31of the charge application portion30so that the surface of the fixing belt11is negatively charged.

In addition, in the present embodiment, a plate-like electrode made of stainless steel on which saw teeth are formed is used as the discharge electrode31of the charge application portion30. However, the present disclosure is not limited to this, but, for example, a tungsten wire may be used as the discharge electrode31.

In addition, in the present embodiment, the charge application portion30is provided as a contactless charger that electrically charges the fixing belt11. However, the present disclosure is not limited to this. For example, the present disclosure can be applied to a case of having a contactless static eliminator (an example of the charge adjustment portion and the contactless static eliminator of the present disclosure), such as a discharging brush, that eliminates static electricity from the fixing belt11. In this case, the static eliminator is attached to the induction heating unit20. This allows a distance between the fixing belt11and the static eliminator to become uniform along the width direction, and makes it possible to uniformly eliminate static electricity from the fixing belt11.

In addition, in the present embodiment, the fixing belt11is heated by the induction heating unit20. However, the present disclosure is not limited to this configuration, but, for example, is applicable to a configuration where the fixing roller is heated by the induction heating unit20.