Patent Description:
In recent years, some image forming apparatuses are configured to print an image, for example, by forming a toner image (also referred to as a developer image) using toner (also referred to as a developer) in a developing unit, transferring the toner image to a sheet (also referred to as a medium), and then fixing the toner image to the sheet by application of heat and pressure in a fixing device. The fixing device includes, for example, a roller and an annular belt which are provided below and above a sheet conveyance path. The fixing device is configured to nip the sheet at a nip portion formed between the roller and the belt and apply heat and pressure to the sheet.

There is a case where the imaging forming apparatus prints an image on a sheet with relatively large irregularities formed on its surface beforehand, such as so-called embossed paper. However, this type of sheet has a low toner fixability, especially at depressions. Thus, there is a proposed technique that improves the toner fixability by specifying the indentation depth of a fixing belt (see, for example, <CIT> (<FIG> and the like)).

Document <CIT> relates to a rotatable fixing member for use in an image forming apparatus such as a copying machine or a printer, a manufacturing method of the rotatable fixing member, and a fixing device.

There is a case where the image forming apparatus prints an image on a sheet with enhanced surface smoothness, such as so-called glossy paper. The glossy paper includes, for example, a base sheet and a resin layer laminated thereon. The resin layer enables the printed image to have high gloss.

However, the glossy paper has fine irregularities formed on a surface of the base sheet, and these fine irregularities may appear on a surface of the resin layer. Thus, the fixing device may fail to appropriately fix the toner to the glossy paper at areas where the fine irregularities are formed, even when the indentation depth of the fixing belt is set in the specified range as described above. As a result, in the image forming apparatus, part of the printed image may have no gloss. In other words, gloss unevenness may occur, and image quality may be degraded.

The present disclosure is made in consideration of the points described above, and an object of the present disclosure is to provide a fixing device and an image forming apparatus that improve the quality of an image fixed to a medium with fine irregularities formed on its surface.

The present invention is defined by the independent claim.

In the present disclosure, the ratio of the first hardness value to the second hardness value of the annular belt is appropriately specified, and thus the annular belt can deform to fit fine irregularities on the medium within a passage time during which the medium passes through the nip region together with the annular belt. Consequently, the developer adhering to both of planar parts and fine irregularities on the medium can be uniformly fixed to the medium, and thus an image with uniform gloss can be formed.

Therefore, the present disclosure can achieve the fixing device and the image forming apparatus that improve the quality of an image fixed to a medium with fine irregularities formed on its surface.

The embodiment and modifications of the present disclosure will be described below with reference to the drawings.

As illustrated in <FIG>, an image forming apparatus <NUM> is an electrophotographic printer and is capable of forming (printing) a color image on a medium M such as plain paper or coated paper. For example, the image forming apparatus <NUM> is a single function printer (SFP) that has only a printer function, and does not have an image scanning function of reading a document, a communication function using a telephone line, or the like.

The image forming apparatus <NUM> includes various components disposed inside a housing <NUM> having a substantially box shape. In the following description, a right end portion in <FIG> is defined as a front of the image forming apparatus <NUM>. Further, a vertical direction, a left-right direction, and a front-back direction are defined as those when the image forming apparatus <NUM> is viewed facing its front. The image forming apparatus <NUM> is capable of printing on a medium M of A3 size at maximum. The image forming apparatus <NUM> forms an image while conveying the medium M of A3 size along a conveyance path described later so that the short sides of the medium M are oriented in the left-right direction. Thus, each part of the image forming apparatus <NUM> has a length corresponding to the short sides of A3 size (<NUM> [mm]) in the left-right direction.

The image forming apparatus <NUM> is controlled entirely by a controller <NUM>. The controller <NUM> is connected to a host device such as a not shown computer. Upon receiving a printing instruction or printing data from the host device, the controller <NUM> executes an image forming process (also referred to as a printing process) to form a printed image on a surface of the medium M.

An operation panel <NUM>, which displays various information and accepts operation inputs, is provided near the front of the upper surface of the housing <NUM>. The operation panel <NUM> has light emitting diodes (LEDs), a touch panel including a combination of a display panel such as a liquid crystal panel and a touch sensor, and the like. Under the control of the controller <NUM>, the operation panel <NUM> displays various information and accepts operation inputs from the user.

A tray <NUM> in which the media M are stored is provided at the bottom in the housing <NUM>. The tray <NUM> is capable of storing the media M of A3 size at maximum with its short sides are oriented in the left-right direction. A feeding conveyance unit <NUM> is provided on the upper front side of the tray <NUM>. The feeding conveyance unit <NUM> has conveyance guides <NUM> provide to face each other with a predetermined interval therebetween. The conveyance guides <NUM> form a feeding conveyance path W1, which is a route for conveying the medium M.

In the feeding conveyance unit <NUM>, a pickup roller <NUM>, a feeding roller <NUM>, a separation roller <NUM>, a registration roller <NUM>, a pressure roller <NUM>, a pair of conveyance rollers <NUM>, and the like are arranged along the feeding conveyance path W1. Each roller has a columnar shape with its central axis oriented in the left-right direction and is rotatably supported. A driving force from a not shown feeding motor is transmitted to some of the rollers. The conveyance rollers <NUM> are disposed facing each other across the feeding conveyance path W1, and the conveyance rollers <NUM> are disposed facing each other across the feeding conveyance path W1.

In the feeding conveyance unit <NUM>, the respective rollers rotate under the control of the controller <NUM> to pick up and convey the media M stacked and stored in the tray <NUM> while separating the media M from one another. Specifically, the pickup roller <NUM> draws the medium M from the tray <NUM>. The feeding roller <NUM> advances the medium M drawn from the tray <NUM> by the pickup roller <NUM>, along the feeding conveyance path W1. The separation roller <NUM> separates the topmost medium M from the other media M when a plurality of media M are taken out of the tray <NUM>. The registration roller <NUM> and the pressure roller <NUM> correct the posture (orientation of each side with respect to an advancing direction) of the medium M when the medium M is skewed with respect to the feeding conveyance path W1, and advance the medium M correctly. The pair of conveyance rollers <NUM> convey the medium M along the feeding conveyance path W1, and further send out the medium M obliquely upward and backward.

On the upper rear side of the pair of conveyance rollers <NUM> in the feeding conveyance unit <NUM>, a transfer unit <NUM> is disposed at the lower side, and four developing units <NUM> are disposed above the transfer unit <NUM>. A straight transfer conveyance path W2 is formed between the transfer unit <NUM> and each developing unit <NUM>. The transfer conveyance path W2 is connected to the feeding conveyance path W1 and extending obliquely upward and backward.

The transfer unit <NUM> includes a drive roller <NUM>, an idle roller <NUM>, a transfer belt <NUM>, four transfer rollers <NUM>, and the like. Each of the drive roller <NUM>, the idle roller <NUM>, and the transfer rollers <NUM> has a columnar shape with its central axis oriented in the left-right direction and is rotatably supported.

The drive roller <NUM> is disposed on the relatively rear side and can be rotated by a driving force supplied from a not shown drive power source. The idle roller <NUM> is disposed at the lower front side of the drive roller <NUM> and slightly away from the drive roller <NUM>. That is, the idle roller <NUM> is disposed near the pair of conveyance rollers <NUM>. The transfer rollers <NUM> are arranged at substantially equal intervals between the drive roller <NUM> and the idle roller <NUM>.

The transfer belt <NUM> is a flexible, endless belt, and is stretched around the drive roller <NUM>, the idle roller <NUM>, and the transfer rollers <NUM>. An upper portion of the transfer belt <NUM> is stretched straightly along the transfer conveyance path W2. Uppermost portions of the transfer rollers <NUM> are in contact with an inner circumferential side of the upper portion of the transfer belt <NUM>. Thus, in the transfer unit <NUM>, when the drive roller <NUM> rotates counterclockwise in <FIG>, it causes the transfer belt <NUM> to move and causes the idle roller <NUM> and the transfer rollers <NUM> to rotate. The upper portion of the transfer belt <NUM> moves obliquely upward and backward along the transfer conveyance path W2.

The four developing units <NUM> (<NUM>, 30Y, <NUM> and 30C) are also referred to as image forming units, and are arranged along the transfer conveyance path W2 above the transfer unit <NUM>, i.e., in the oblique direction from the lower front side to the upper rear side. The developing units <NUM> respectively correspond to black (K), yellow (Y), magenta (M), and cyan (C). The developing units <NUM> are configured in the same manner except for color.

The developing unit <NUM> includes a development processing unit <NUM> and an exposure processing unit <NUM>. The development processing unit <NUM> has a toner storage unit that stores a toner as a developer, a plurality of rollers, a photosensitive drum <NUM>, and the like. Among these components, each of the rollers and the photosensitive drum <NUM> has a columnar or cylindrical shape with its central axis oriented in the left-right direction and is rotatably supported. The photosensitive drum <NUM> is located at the lowermost side in the development processing unit <NUM> and is in contact with the transfer belt <NUM> so as to nip the transfer belt <NUM> between the transfer roller <NUM> and the photosensitive drum <NUM>.

The exposure processing unit <NUM> includes a plurality of LEDs arranged in the left-right direction above the corresponding photosensitive drum <NUM>. In the exposure processing unit <NUM>, the LEDs emit light under the control of the controller <NUM> to expose an outer circumferential surface of the photosensitive drum <NUM>, thereby forming an electrostatic latent image. The development processing unit <NUM> causes the toner to adhere to the outer circumferential surface of the photosensitive drum <NUM> to form a toner image (also referred to as a developer image).

When the medium M is conveyed along the transfer conveyance path W2, the transfer unit <NUM> transfers the toner image from the photosensitive drum <NUM> to the medium M and causes the toner image to adhere to the surface of the medium M.

A fixing unit <NUM> as a fixing device (i.e., fuser) is disposed on the rear side of the transfer unit <NUM>, i.e., on the rear side of the rearmost developing unit 30C. The fixing unit <NUM> fixes the toner image to the surface of the medium M by applying heat and pressure to the medium M while conveying the medium M along a fixing conveyance path W3 and sends the medium M obliquely upward and backward (to be described in detail).

A double-sided printing unit <NUM> is located at the lower side and rear side of the fixing unit <NUM>. The double-sided printing unit <NUM> includes a switching unit <NUM> provided on the rear side of the fixing unit <NUM>, a plurality of conveyance guides, a plurality of pairs of conveyance rollers, and the like. These components form a circulation conveyance path W4, a temporary evacuation conveyance path W5, and the like. The circulation conveyance path W4 is formed to connect the switching unit <NUM> and the pair of conveyance rollers <NUM> of the feeding conveyance unit <NUM>.

When performing double-sided printing, the double-sided printing unit <NUM> switches the switching unit <NUM> under the control of the controller <NUM> to advance the medium M to the temporary evacuation conveyance path W5. Subsequently, after the end of the medium M passes through the switching unit <NUM>, the double-sided printing unit <NUM> reverses the advancing direction of the medium M, and advances the medium M along the circulation conveyance path W4. The circulation conveyance path W4 connects to the feeding conveyance path W1 of the feeding conveyance unit <NUM> at the vicinity of the pair of conveyance rollers <NUM>. As a result, the double-sided printing unit <NUM> enables the medium M to advance from the feeding conveyance path W1 to the transfer conveyance path W2 again in a state where the medium M is turned over, so that an image can be transferred to the back side of the medium M. In this regard, the double-sided printing unit <NUM> advances the medium M obliquely upward and backward when double-sided printing is not performed on the medium M and when the image is transferred to the back side of the medium M.

An ejection conveyance unit <NUM> is disposed on the upper rear side of the switching unit <NUM>. The ejection conveyance unit <NUM> has a configuration partially similar to a part of the feeding conveyance unit <NUM>. The ejection conveyance unit <NUM> has conveyance guides <NUM> facing each other with a predetermined interval therebetween. The conveyance guides <NUM> form a sheet ejection conveyance path W6, which is a route for conveying a medium M. An ejection port <NUM> is formed at the end of the path W6. The ejection conveyance unit <NUM> includes pairs of conveyance rollers <NUM> and <NUM> and the like arranged in this order along the sheet ejection conveyance path W6.

In the ejection conveyance unit <NUM>, the pairs of conveyance rollers <NUM> and <NUM> are rotated under the control of the controller <NUM> to convey the medium M, which has been introduced from the fixing unit <NUM> via the switching unit <NUM>, along the sheet ejection conveyance path W6 and to eject the medium M through the ejection port <NUM>. The ejected medium M is placed on an ejection tray <NUM> formed on a top surface of the housing <NUM>.

In this way, the image forming apparatus <NUM> conveys the medium M along the conveyance paths W, transfers a toner image formed by the developing unit <NUM> to the medium M, and then fixes the toner image to the medium M in the fixing unit <NUM>. Thus, the image forming apparatus <NUM> is able to form an image, i.e., to perform printing.

Next, the configuration of the fixing unit <NUM> will be described. <FIG> is a schematic perspective view of the fixing unit <NUM>. <FIG> is a schematic cross-sectional view of the fixing unit <NUM>. As illustrated in <FIG>, the fixing unit <NUM> has a rectangular parallelepiped shape as a whole elongated in the left-right direction.

The fixing unit <NUM> is configured so that a plurality of components are incorporated inside a hollow fixing housing <NUM> that has a rectangular parallelepiped shape. On the front side and the back side of the fixing housing <NUM>, elongated holes that are elongated in the left-right direction and penetrate the fixing housing <NUM> in the front-back direction are formed. The elongated holes allow the medium M to pass through the fixing housing <NUM>.

Inside the fixing housing <NUM>, a heating section <NUM> is disposed on the upper side, and a pressurizing section <NUM> is disposed on the lower side. The heating section <NUM> has a columnar shape as a whole with its central axis oriented in the left-right direction. The heating section <NUM> is supported by the fixing housing <NUM> so that the heating section <NUM> is displaceable in the substantially vertical direction.

As illustrated in <FIG>, the heating section <NUM> mainly includes a heating central portion <NUM> located at its center and a heating belt <NUM> provided to surround the heating central portion <NUM>. The heating central portion <NUM> has a hollow rectangular parallelepiped shape as a whole that is elongated in the left-right direction. The heating central portion <NUM> includes support bodies <NUM> and <NUM>, a heat transfer plate <NUM>, a heater <NUM>, a partition plate <NUM>, a temperature sensor <NUM>, and the like.

The support body <NUM> is a molded part and is made of, for example, a heat-resistant resin material. The support body <NUM> is in the form of a hollow quadrangular prism extending in the left-right direction as a whole with its top surface removed. The support body <NUM> is formed, for example, by bending a plate-shaped metal member. The support body <NUM> is in the form of a hollow quadrangular prism extending in the left-right direction as a whole with its bottom surface removed. A front side plate of the support body <NUM> is in contact with the front side of a front side plate of the support body <NUM>. A back side plate of the support body <NUM> is in contact with the back side of a back side plate of the support body <NUM>. Thus, the support bodies <NUM> and <NUM> are combined to each other to form one quadrangular prism extending in the left-right direction as a whole.

The heat transfer plate <NUM> has a plate shape that is elongated in the left-right direction and thin in the vertical direction. The heat transfer plate <NUM> is located below a lower plate of the support body <NUM>. The heat transfer plate <NUM> is made of a metal material with relatively high thermal conductivity, such as stainless steel, for example. The heat transfer plate <NUM> efficiently transfers heat generated by the heater <NUM> to be described later. The heater <NUM> as a heating member has a plate shape that is elongated in the left-right direction and thin in the vertical direction. The heater <NUM> is located below the support body <NUM>. The heater <NUM> generates heat by electric power supplied from a predetermined power supply unit under the control of the controller <NUM> (<FIG>).

The partition plate <NUM> mainly includes a lower plate having a plate shape that is elongated in the left-right direction and thin in the vertical direction. The partition plate <NUM> also includes a front plate and a back plate which are bent upward from the front and back sides of the lower plate, respectively. In the partition plate <NUM>, the lower plate is located under the heater <NUM> to separate the heater <NUM> from the heating belt <NUM>, so that the heater <NUM> and the heating belt <NUM> do not come into direct contact with each other.

The temperature sensor <NUM> is located above the lower plate inside the support body <NUM>. The temperature sensor <NUM> detects the temperature of the heater <NUM> via the heat transfer plate <NUM>, generates an electrical signal corresponding to the detected temperature, and sends the electrical signal to the controller <NUM> (<FIG>). The controller <NUM> controls electric power to be supplied to the heater <NUM> based on the electric signal from the temperature sensor <NUM>, thereby adjusting the temperature of the heater <NUM> to a desired temperature.

The heating belt <NUM> as an annular belt is an endless belt that has a hollow cylindrical shape and has a sufficient length in the left-right direction. The heating belt <NUM> is provided to move around the heating central portion <NUM>. As illustrated in a schematic cross-sectional view of <FIG>, the heating belt <NUM> has a layered structure in which three types of members, namely, a base <NUM>, an elastic layer <NUM>, and a surface layer <NUM>, are stacked in this order.

The base <NUM> is located at the innermost side of the heating belt <NUM> and is made of polyimide, for example. The thickness of the base <NUM> can be approximately <NUM> to <NUM> [µm]. In this embodiment, the thickness of the base <NUM> is set to approximately <NUM> to <NUM> [µm]. The base <NUM> can also be made of a metal material. In this case, the thickness of the base <NUM> can be approximately <NUM> to <NUM> [µm].

The elastic layer <NUM> is located between the base <NUM> and the surface layer <NUM> and is made of silicone rubber, for example. The thickness of the elastic layer <NUM> can be approximately <NUM> to <NUM> [µm], and is approximately <NUM> to <NUM> [µm] in this embodiment. The hardness of silicone rubber that forms the elastic layer <NUM> is desirably approximately <NUM> to <NUM> [degree] using a durometer type A (Shore A) measurement method based on JIS K <NUM>. In this embodiment, a material having a hardness of approximately <NUM> to <NUM> [degree] is used for the elastic layer <NUM>.

The surface layer <NUM> is located on the outermost side of the heating belt <NUM> and is made of, for example, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA). The thickness of the surface layer <NUM> can be approximately <NUM> to <NUM> [µm]. In this embodiment, the thickness of the surface layer <NUM> is set to approximately <NUM> to <NUM> [µm]. The surface layer <NUM> forms an outer circumferential surface of the heating belt <NUM>.

In this regard, a liquid lubricant is applied to an inner circumferential surface of the heating belt <NUM>. Thus, the liquid lubricant is interposed between the partition plate <NUM> and the heating belt <NUM>. Consequently, the heating belt <NUM> can slide smoothly with respect to the partition plate <NUM>.

The pressurizing section <NUM> (<FIG> and <FIG>) as a facing member is formed as a whole to have a columnar shape with its central axis oriented in the left-right direction. The diameter of the pressurizing section <NUM> is approximately <NUM> [mm]. The pressurizing section <NUM> is also referred to as a pressurizing roller. As illustrated in a schematic cross-sectional view of <FIG>, the pressurizing section <NUM> has an elastic layer <NUM>, a primer layer <NUM>, and a surface layer <NUM> that are stacked on a central material <NUM> in this order.

The central material <NUM> as a center shaft is made of free-cutting steel (also referred to as SUM), for example. The central material <NUM> has a columnar shape with its central axis oriented in the left-right direction. The diameter of the central material <NUM> is approximately <NUM> [mm]. The elastic layer <NUM> is made of, for example, silicone rubber. The elastic layer <NUM> is formed on the outer circumferential surface of the central material <NUM> so that the thickness of the elastic layer <NUM> is approximately <NUM> [mm]. The primer layer <NUM> is made of, for example, a non-conductive RTV (Room Temperature Vulcanizing) silicone rubber. The primer layer <NUM> is formed on the outer circumferential surface of the elastic layer <NUM> so that the thickness of the primer layer <NUM> is approximately <NUM> [µm] or less. The surface layer <NUM> is made of, for example, a non-conductive PFA, and the thickness of the surface layer <NUM> is <NUM> to <NUM> [µm].

In addition, compression springs <NUM> are provided on the left and right sides in the fixing housing <NUM> of the fixing unit <NUM> (<FIG> and <FIG>). The compression springs <NUM> are coil springs, and bias the heating section <NUM> toward the pressurizing section <NUM> via not shown components.

In the fixing unit <NUM>, a pressure corresponding to a load of <NUM> to <NUM> [kg] is desirably applied between the heating section <NUM> and the pressurizing section <NUM> due to the weight of the heating section <NUM>, the biasing force of the compression spring <NUM>, and the like. In this embodiment, the fixing unit <NUM> is operated in a state where a pressure corresponding to a load of <NUM> [kg] is applied between the heating section <NUM> and the pressurizing section <NUM>.

Thus, in the fixing unit <NUM>, the heating section <NUM> is pressed against the pressurizing section <NUM>, and the pressurizing section <NUM> is elastically deformed to form a nip region N between the heating belt <NUM> and the pressurizing section <NUM>. Inside the image forming apparatus <NUM> (<FIG>), the fixing conveyance path W3 is formed along the nip region N. In the fixing unit <NUM>, a nip width WN (<FIG>), which is the length of the nip region N in the conveyance direction (i.e., substantially the front-back direction), is set to <NUM> to <NUM> [mm].

When the image forming apparatus <NUM> performs the printing process, the fixing unit <NUM> causes the heater <NUM> of the heating section <NUM> to generate heat and rotates the pressurizing section <NUM> to cause the heating belt <NUM> to circulate. In the fixing unit <NUM>, when the medium M is conveyed along the fixing conveyance path W3, the medium M is nipped between the heating belt <NUM> and the pressurizing section <NUM> at the nip region N. At this time, the fixing unit <NUM> fixes the toner by applying heat and pressure thereto while moving the heating belt <NUM> at the same speed as the medium M in a state where the heating belt <NUM> is in contact with the medium M.

In the image forming apparatus <NUM>, a highly resistant sheet such as so-called coated paper or water-resistant paper may be used. The highly resistant sheet is a sheet with which relatively high resistance is generated during feeding. Specifically, coated paper such as "Kaleka (registered trademark)" manufactured by Kokusai Pulp & Paper Co. , "Lamifree (registered trademark)" manufactured by Nakagawa Mfg. , or "Eco Crystal (registered trademark)" manufactured by Tomoegawa Seisakusho Co. , or the like can be used. In this case, the image forming apparatus <NUM> reduces the conveyance speed of the medium M (i.e., the feeding speed) as compared to the case where plain paper is used, thereby enhancing the fixing efficiency of the toner to the medium M and improving the toner fixability.

In the image forming apparatus <NUM>, the conveyance speed when using coated paper or the like can be set to approximately <NUM> to <NUM> [mm/s], and the nip width WN can be set to <NUM> to <NUM> [mm]. In this embodiment, the conveyance speed is set to <NUM> [mm/s], and the nip width WN is set to <NUM> [mm]. Thus, in the image forming apparatus <NUM>, a passage time required for a predetermined point (i.e., a predetermined position) on the medium M to pass through the nip region N in the fixing unit <NUM> is approximately <NUM> [s].

In this embodiment, for example, when the conveyance speed of the medium M is <NUM> [mm/s] and the nip width WN is <NUM> [mm], the passage time required for a predetermined point of the medium M to pass through the nip region N is <NUM> [s]. For example, when the conveyance speed of the medium M is <NUM> [mm/s] and the nip width WN is <NUM> [mm], the passage time required for a predetermined point of the medium M to pass through the nip region N is <NUM> [s].

The coated paper or the like described above has a structure in which a relatively thin surface layer of resin or the like is laminated on the surface of a base material made of paper (i.e., cellulose or the like). In the coated paper, relatively small irregularities formed on the surface of the base material are filled with the surface layer, so that the surface of the coated paper is formed more smoothly than plain paper. Thus, when an image is printed on the coated paper by the image forming apparatus <NUM> or the like, the printed image is expected to be uniformly glossy and to be high in quality.

However, in actual coated paper, due to irregularities formed on the base material, fine depressions D may be formed on the surface of the coated paper. The fine depression D has, for example, a circular or oval shape with a diameter or major axis of approximately <NUM> to <NUM> [mm] and a depth of approximately <NUM> [µm].

When a toner image is fixed to the medium M composed of such coated paper or the like, the fixing unit <NUM> desirably causes the heating belt <NUM> to partially deform to fill in the depressions D while the depressions D pass through the nip region N. This causes the surface of the heating belt <NUM> to come into contact with the inner surfaces of the depressions D so that toner is pressed against the surface of the medium M.

Meanwhile, in the image forming apparatus <NUM>, as described above, when coated paper or the like is used as the medium M, the conveyance speed of the medium M is set to <NUM> [mm/s], and accordingly the passage time of the medium M through the nip region N of the fixing unit <NUM> is approximately <NUM> [s]. This means that, when the heating belt <NUM> deforms along a contour corresponding to the depressions D within <NUM> [s] after the heating belt <NUM> contacts the medium M and starts deforming, heat and pressure can be appropriately applied to the medium M. In other words, in the image forming apparatus <NUM>, if the deformation speed, hardness and the like of the heating belt <NUM> of the fixing unit <NUM> are set within appropriate ranges, toner can be appropriately fixed even in the depressions D, and gloss can be given to an image.

The relationship between the deformation speed or hardness of the heating belt <NUM> and the followability of the heating belt <NUM> to the medium M will be described with reference to <FIG> are schematic cross-sectional views illustrating the states in which the heating belt <NUM> contacts the surface of the medium M with the depressions D formed thereon at the nip region N.

For example, as illustrated in <FIG>, in a case where the hardness of the heating belt <NUM> is relatively low and the deformation speed of the heating belt <NUM> is relatively slow, the heating belt <NUM> cannot completely fill in the depressions D, and thus heat and pressure cannot be sufficiently transferred to the toner at the inner surfaces of the depressions D. In other words, the heating belt <NUM> exhibits low followability to the contour of the medium M having the depressions D or exhibits unsatisfactory responsiveness. Thus, the heating belt <NUM> cannot sufficiently follow the medium M within the passage time. In this case, gloss is not given to areas where the depressions D are formed, i.e., gloss unevenness occurs. Thus, image quality is evaluated to be low.

In contrast, as illustrated in <FIG>, in a case where the hardness of the heating belt <NUM> is within an appropriate range and the deformation speed of the heating belt <NUM> is within an appropriate range, the heating belt <NUM> can completely fill in the depressions D, and thus heat and pressure can be sufficiently transferred to the toner at the inner surfaces of the depressions D. In other words, the heating belt <NUM> exhibits high followability to the contour of the medium M having the depressions D, and exhibits favorable responsiveness. In this case, a sufficient gloss is given to the image even in areas where the depressions D are formed, and uniform gloss can be obtained across the surface of the medium M. Thus, image quality is evaluated to be high.

Furthermore, as illustrated in <FIG>, in a case where the hardness of the heating belt <NUM> is relatively high, the heating belt <NUM> cannot completely fill in the depressions D, and thus heat and pressure cannot be sufficiently transferred to the toner at the inner surfaces of the depressions D. In other words, the heating belt <NUM> exhibits low followability to the contour of the medium M having the depressions D, and exhibits the unsatisfactory responsiveness. In this case, gloss is not given to areas where the depressions D are formed, and gloss unevenness occurs as in the case of <FIG>. Thus, image quality is evaluated to be low.

Thus, in the fixing unit <NUM>, it is thought that when the hardness and deformation speed of the heating belt <NUM> are set within appropriate ranges, the heating belt <NUM> can appropriately follow the contour of the medium M so that the toner can be suitably fixed to every part of the medium M. As a result, the possibility of occurrence of gloss unevenness can be reduced.

In order to measure the hardness of a relatively thin member such as the heating belt <NUM>, a so-called microhardness tester is generally used. In the measurement using the microhardness tester, for example, a probe (also referred to as a measurement terminal) having a columnar shape or the like is brought into contact with a target member and pushed in the target member with a predetermined load or at a predetermined speed, whereby the hardness of the target member is measured based on an amount of displacement of the probe.

In this embodiment, the micro hardness tester "Micro Rubber Hardness Tester MD-1capa" manufactured by Kobunshi Keiki Co. In this embodiment, a probe having a cylindrical shape with a diameter of <NUM> [mm] is used for measurement, the descending speed of the probe (i.e., the pressing speed) is set to <NUM> [mm/s], and the load is set to <NUM> to <NUM> [Nm].

<FIG> is a graph illustrating examples of changes in measured values over time obtained using the microhardness tester for a plurality of heating belts <NUM> having different configurations. The vertical axis represents a hardness value that is converted to a relative value [%] with respect to a hardness value finally saturated (hereafter referred to as a saturation hardness value). The horizontal axis represents an elapsed time from the start of measurement. The elapsed time is plotted every <NUM> [s]. Hereinafter, a characteristic curve connecting the plots shown in <FIG> is also referred to as a profile.

In <FIG>, a manner in which the measured value by the microhardness tester increases as the time elapses after the start of measurement can be observed. In <FIG>, difference in the profile shape depending on the configuration of the heating belt <NUM> can also be observed. The difference in the profile shape of the heating belt <NUM> represents the difference of the deformation speed of the heating belt <NUM>.

Therefore, in this embodiment, the microhardness tester is used to measure the hardness of the heating belt <NUM>. Further, in this embodiment, a measured value at the time when <NUM> [s] has elapsed after the start of the measurement (hereinafter referred to as a hardness value after <NUM>) is regarded as a value corresponding to the deformation speed of the heating belt <NUM>. Hereinafter, <NUM> [s] described above is also referred to as a measurement time.

Further, in this embodiment, the relationship between a hardness value after <NUM> of the heating belt <NUM> and the quality of an image printed on the medium M using the heating belt <NUM> is examined. In order to facilitate the comparison, the hardness value after <NUM> is expressed as a ratio relative to the saturation hardness value which is the hardness value finally converged (hereinafter referred to as a hardness ratio after <NUM>). That is, the hardness value is normalized. For convenience of explanation, the hardness value after <NUM> and the saturation hardness value are also hereinafter referred to as a first hardness value and a second hardness value, respectively. The hardness value after <NUM> is expressed as a ratio (A/B) of the first hardness value (A) to the second hardness value (B).

Specifically, in this embodiment, <NUM> types of heating belts <NUM> (72A to <NUM>) having various elastic layers <NUM> and surface layers <NUM> are prepared, and the hardness of each heating belt <NUM> is measured using the microhardness tester in an evaluation test. <FIG> is a table showing the specifications and measurement results of the respective heating belts <NUM> in the form of a table.

As the specifications of each heating belt <NUM>, <FIG> shows the thickness [µm] of the elastic layer <NUM>, the hardness [degree] of the elastic layer <NUM>, and the thickness [µm] of the surface layer <NUM>. <FIG> also shows measured values of the saturation hardness value [degree], the hardness value after <NUM> [degree] of the heating belt <NUM>, and the hardness ratio after <NUM> calculated based on both measured values. Note that the values of the hardness ratio after <NUM> are rounded to <NUM> decimal place.

Next, in this embodiment, a printing test is performed. In the printing test, a test image described later is printed on coated paper as the medium M using each of the heating belts <NUM> (72A to <NUM>) in the fixing unit <NUM> of the image forming apparatus <NUM>. Then, printed images are evaluated. In the printing test, "C844" manufactured by Oki Electric Industry Co. is used as the image forming apparatus <NUM>.

In the printing test, an image whose entire surface is uniformly black (so-called full solid image) is printed as the test image. When gloss unevenness appears on the medium M after printing, it is thought that fine irregularities are formed on the surface of the medium M and make the surface non-planar. That is, it is thought that the degree of gloss unevenness increases as the area of planar parts of the medium M decreases and the area of non-planar parts of the medium M increases.

For this reason, in this embodiment, as for evaluation of the printing results on the media M, the printing results are classified into multiple levels based on the ratio of the area of planar parts to the entire surface of the medium M. Each classified level has a high correlation with the degree of gloss unevenness. That is, in this embodiment, the ratio of the planar parts to the entire surface of the medium M after the printing is used as an objective index representing the degree of gloss unevenness on the medium M.

Specifically, in this embodiment, the test image is printed on the medium M by the image forming apparatus <NUM> that incorporates each heating belt <NUM> in the fixing unit <NUM>. In this embodiment, "Lamifree (registered trademark)" manufactured by Nakagawa Mfg. is used as the medium M.

Then, in this embodiment, the surface contour of the medium M is observed using a laser microscope, and microscopic images are captured. In this embodiment, a confocal microscope "OPTELICS (registered trademark) HYBRID" manufactured by Lasertec Corporation is used as the laser microscope.

Subsequently, in this embodiment, a binarization process is performed on the microscope image obtained using the laser microscope based on the luminance of each pixel in the microscope image, thereby classifying the image into planar parts and non-planar parts. Further, in this embodiment, the ratio of the area of the planar parts to the area of the entire microscopic image is calculated, and the calculated value is defined as a toner planar area ratio [%]. The settings of the laser microscope are provided below.

Further, in this embodiment, the following thresholds for calculated values of the toner planar area ratio [%] of each heating belt are set. Using the thresholds, calculated values of the toner planar area ratio are classified into five evaluation levels, namely, "Level <NUM>" in which the degree of gloss unevenness is relatively high to "Level <NUM>" in which gloss unevenness is hardly observed. By visually observing the states of gloss unevenness of a plurality of media M having various toner planar area ratios [%], the thresholds for the respective evaluation levels are appropriately set so that differences in the gloss unevenness between the respective levels are significantly noticeable.

Such an evaluation test is conducted using each heating belt <NUM>, and evaluation levels are obtained as shown in <FIG>. <FIG> is a graph obtained by plotting the evaluation levels of the respective heating belts <NUM>. In <FIG>, the horizontal axis represents the hardness ratio after <NUM> and the vertical axis represents the evaluation level. In <FIG>, plots where the evaluation level is Level <NUM> or higher are indicated by the symbol "O" (good), and plots where the evaluation level is Level <NUM> or lower are indicated by the symbol "X" (poor). Hereinafter, the correlation between the hardness ratio after <NUM>, the evaluation level in the evaluation test and the like will be described with reference to <FIG> and <FIG>.

In this evaluation test, when a value of the hardness ratio after <NUM> is within a range R1 of <NUM> (<NUM>%) to <NUM> (<NUM>%), the evaluation level is Level <NUM> or higher. It is thought that in the fixing unit <NUM>, the heating belt <NUM> exhibits relatively quick responsiveness to the pressing and exhibits high followability to the depressions D formed on the medium M as illustrated in <FIG>. Thus, the image forming apparatus <NUM> can uniformly apply heat and pressure to every part of the medium M at the nip region N of the fixing unit <NUM>, so that gloss unevenness in the image printed on the medium M can be suitably reduced. In this case, the hardness of the elastic layer <NUM> is within a range of <NUM> to <NUM> [degree], and the hardness value after <NUM> is within a range of <NUM> to <NUM> [degree].

In addition, in this evaluation test, when a value of the hardness ratio after <NUM> [%] is within a range R2 of <NUM> (<NUM>%) to <NUM> (<NUM>%), the evaluation level is Level <NUM>. It is thought that in the fixing unit <NUM>, the heating belt <NUM> exhibits further quick responsiveness to the pressing, and exhibits higher followability to the depressions D formed on the medium M. Thus, the image forming apparatus <NUM> can significantly reduce gloss unevenness in the image printed on the medium M, and remarkably high image quality can be obtained.

On the other hand, in this evaluation test, when a value of the hardness ratio after <NUM> is less than <NUM>, the evaluation level is Level <NUM> or lower. It is thought that in the fixing unit <NUM>, the heating belt <NUM> exhibits relatively slow responsiveness to the pressing, and exhibits low followability to the depressions D formed on the medium M as illustrated in <FIG>. As a result, the image forming apparatus <NUM> causes more gloss unevenness in the image printed on the medium M.

In addition, in this evaluation test, when a value of the hardness ratio after <NUM> [%] is more than <NUM>, the evaluation level is Level <NUM> or lower. It is thought that in the fixing unit <NUM>, the heating belt <NUM> is relatively hard, and the elastic deformability of the heating belt <NUM> is relatively low, so that the heating belt <NUM> exhibits low followability to the depressions D formed on the medium M as illustrated in <FIG>. As a result, the image forming apparatus <NUM> causes more gloss unevenness in the image printed on the medium M.

In this way, this evaluation test shows that the degree of gloss unevenness in the image printed on the medium M changes depending on the value of the hardness ratio after <NUM>. This evaluation test also shows the range of the hardness ratio after <NUM> and the range of the hardness value after <NUM> with which the degree of gloss unevenness can be suitably reduced.

Based on the above, the fixing unit <NUM> of the image forming apparatus <NUM> according to this embodiment includes the heating belt <NUM> in which the value of the hardness ratio after <NUM> is at least within the range R1 of <NUM> to <NUM>, and preferably within the range R2 of <NUM> to <NUM>.

With the configuration described above, in the image forming apparatus <NUM> of this embodiment, the heating belt <NUM> of the fixing unit <NUM> sufficiently deforms within a time during which the medium M passes through the nip region N in the case of printing an image on the medium M of coated paper. Specifically, the image forming apparatus <NUM> has the heating belt <NUM> in which the value of the hardness ratio after <NUM> measured by the microhardness tester is at least within the range R1 of <NUM> to <NUM>.

Thus, in the image forming apparatus <NUM>, the heating belt <NUM> can sufficiently deform to fit the depressions D on the medium M within approximately <NUM> [s] during which the medium M passes through the nip region N of the fixing unit <NUM>. Thus, the heating belt <NUM> can be brought into contact with the surfaces of the depressions D (<FIG>). Thus, the image forming apparatus <NUM> can sufficiently fix the toner to both of planer parts and the depressions D on the medium M by applying heat and pressure using the heating belt <NUM>. Therefore, uniform gloss can be given to the image printed on the medium M without any gloss unevenness.

In the image forming apparatus <NUM>, when the hardness value after <NUM> of the heating belt <NUM> of the fixing unit <NUM> is <NUM> [degree] or more and <NUM> [degree] or less, the evaluation level is Level <NUM> or higher (<FIG>), and thus gloss unevenness can be suitably suppressed.

Furthermore, the image forming apparatus <NUM> may also include the heating belt <NUM> of the fixing unit <NUM> in which the value of the hardness ratio after <NUM> is within the range R2 of <NUM> to <NUM>. In this case, the image forming apparatus <NUM> can more suitably fix the toner to both of planer parts and the depressions D on the medium M by the heating belt <NUM>. Thus, gloss unevenness in the image printed on the medium M can be more suitably suppressed, and gloss can be more suitably given to the image printed on the medium M.

In particular, in this embodiment, among measured values by the microhardness tester, the hardness value after <NUM> (i.e., the value at the time when <NUM> [s] has elapsed after the start of measurement) is used in addition to the saturation hardness value. In this embodiment, <NUM> [s] is regarded as the time required for the medium M to pass through the nip region N. Specifically, this time is calculated based on the conveyance speed of the medium M and the nip width WN which is the length of the nip region N. Thus, in the image forming apparatus <NUM>, it is possible to employ the appropriate heating belt <NUM> that completely finishes deforming to fit the depressions D within a time during which the medium M passes through the nip region N.

Furthermore, it is confirmed in this embodiment that as long as a value of the hardness ratio after <NUM> is at least within the range R1 of <NUM> to <NUM>, gloss unevenness can be suitably suppressed by modifying the conveyance speed and the nip width WN even if the passage time required for the medium to pass through the nip region N is changed to a time between <NUM> [s] and <NUM> [s].

Therefore, when a value of the hardness ratio of the heating belt <NUM> reaches a value of <NUM> or more and <NUM> or less until the medium M finishes passing through the nip region N, gloss unevenness can be suppressed in both of the planer parts and the depressions D on the medium M. That is, regarding the condition required for the heating belt <NUM>, an elapsed time after the start of measurement by the microhardness tester and the passage time through the nip region N are not necessarily identical to each other. For example, the condition required for the heating belt <NUM> may be set so that a value of the hardness ratio is within the range R1 of <NUM> to <NUM> at a time when <NUM> ± <NUM> [s] has elapsed after the start of measurement by the microhardness tester.

From another viewpoint, in this embodiment, the microhardness tester is used by a method partially different from the normal method. In the normal method using the microhardness tester, a probe is pressed against a target to be measured, and when a certain period of time has elapsed and a measured value becomes stable, the measured value at this time (i.e., the saturation hardness value) is used as the measurement value.

In contrast, in this embodiment, a change in the heating belt <NUM> over time caused by being pressed by the probe of the microhardness tester is considered to be very close to a change in the heating belt <NUM> over time caused by contact with the depressions D on the medium M. Thus, in this embodiment, the change in the contour of the heating belt <NUM> over time can be grasped by sequentially reading the change in the measured value over time by the microhardness tester.

From a further viewpoint, the image forming apparatus <NUM> is configured so that the nip width WN of the nip region N is as large as possible in order to suitably fix the toner to the medium M. Specifically, in the fixing unit <NUM> (<FIG>), the heating section <NUM> is not in the form of a roller such as the pressurizing section <NUM>, but is in the form of the heating belt <NUM> that moves around the heating central portion <NUM>. The heater <NUM>, the partition plate <NUM>, and the like which are provided in the heating central portion <NUM> have sufficient length in the front-back direction. In the fixing unit <NUM> configured in this way, the heating belt <NUM> that moves around the heating central portion <NUM> is required to be formed relatively thin, and it is difficult for the heating belt <NUM> to have a sufficient thickness. As a result, it is conventionally difficult to select an appropriate hardness of the heating belt <NUM>.

From this point, in this embodiment, the favorable ranges R1 and R2 (<FIG>) are specified by using the harness ratio after <NUM> as an index while focusing on the followability and responsiveness of the heating belt <NUM> within the passage time during which the medium passes through the nip region N (i.e., <NUM> [s]). Consequently, in the image forming apparatus <NUM>, the followability and responsiveness of the heating belt <NUM> formed relatively thin can be enhanced appropriately, while securing a relatively large nip width WN in the fixing unit <NUM> (<FIG>). Thus, favorable gloss can be given to the formed image.

In this embodiment, the toner planar area ratio based on the luminance of each pixel in the microscope image is used as an index, and the evaluation level is classified according to the value of the toner planar area ratio. Thus, in this embodiment, regarding the presence or absence and the degree of gloss unevenness, an objective evaluation level can be appropriately determined for each heating belt <NUM> based on clear classification according to the uniform standards, rather than ambiguous classification depending on visual observation. As a result, in the image forming apparatus <NUM>, an image having sufficient gloss such as an image with little gloss unevenness can be printed on the medium M by using the appropriate heating belt <NUM> selected based on the appropriately evaluated level.

With the configuration described above, when an image is printed on the medium M of the coated paper, the heating belt <NUM> of the fixing unit <NUM> in the image forming apparatus <NUM> is configured so that a value of its hardness ratio after <NUM> measured using the microhardness tester is at least within the range R1 of <NUM> to <NUM>. Thus, in the image forming apparatus <NUM>, the heating belt <NUM> can sufficiently deform to fit the depressions D on the medium M within approximately <NUM> [s] during which the medium M passes through the nip region N of the fixing unit <NUM>. Thus, the image forming apparatus <NUM> can sufficiently fix the toner to both the planer parts and the depressions D on the medium M. Therefore, gloss unevenness in the image printed on the medium M can be suppressed, and uniform gloss can be given to the image printed on the medium M.

In the above-described embodiment, the nip width WN of the fixing unit <NUM> is <NUM> to <NUM> [mm], the conveyance speed of the medium M is <NUM> [mm/s], and the passage time required for a predetermined point on the medium M to pass through the nip region N is approximately <NUM> [s]. Accordingly, the heating belt <NUM> is evaluated using the hardness value after <NUM> and the hardness ratio after <NUM>. However, the present disclosure is not limited thereto, and the passage time may be set to various times, for example, <NUM> [s], <NUM> [s], or the like, by setting the nip width WN in the fixing unit <NUM> and the conveyance speed of the medium M to various values. In this case, in accordance with the passage time, the hardness or hardness ratio at the time when the passage time has elapsed after the start of measurement may be used. Alternatively, the hardness or hardness ratio at a time shorter than the passage time may be used.

In the above-described embodiment, the toner planar area ratio is calculated based on the luminance value of the microscopic image obtained using the laser microscope in the evaluation test of the heating belt <NUM>, and the heating belt <NUM> is evaluated by classification into the five evaluation levels (Level <NUM> to Level <NUM>) using the calculated toner planar area ratio. However, the present disclosure is not limited thereto. The classification into respective evaluation levels may be carried out by using various methods, such as the subjective classification based on the evaluator's visual inspection, for example. The number of evaluation levels for the classification is not limited to five, but may be four or less, or six or more.

Further, in the above-described embodiment, the thickness of the base <NUM> in the heating belt <NUM> is approximately <NUM> to <NUM> [µm], the thickness of the elastic layer <NUM> is approximately <NUM> to <NUM> [µm], and the thickness of the surface layer <NUM> is approximately <NUM> to <NUM> [µm] (<FIG>). However, the present disclosure is not limited thereto, and the thicknesses of the base <NUM>, elastic layer <NUM>, and surface layer <NUM> may be set to other respective values.

Furthermore, in the above-described embodiment, the hardness of the elastic layer <NUM> in the heating belt <NUM> is <NUM> to <NUM> [degree]. However, the present disclosure is not limited thereto, and the hardness of the elastic layer <NUM> may be set to other various values.

Moreover, in the above-described embodiment, the pressurizing section <NUM> of the fixing unit <NUM> (<FIG>, <FIG> and <FIG>) is configured as a pressure roller that has the elastic layer <NUM> or the like formed on the outer circumferential surface of the central material <NUM>. However, the present disclosure is not limited thereto, and the pressurizing section <NUM> may be configured in various ways. For example, it is possible to employ a combination of a heating central portion and a heating belt as in the heating section <NUM>.

In addition, the above-described embodiment uses the medium M, such as coated paper, that has the surface layer of resin or the like provided on the surface of paper as the base material with fine depressions D formed on the surface of the surface layer. However, the present disclosure is not limited thereto and may use various media with fine depressions or irregularities formed on their surfaces as in coated paper.

Furthermore, in the above-described embodiment, by using the microhardness tester, a columnar probe is pressed into the heating belt <NUM> as the target to be measured, and the hardness of the heating belt <NUM> is measured based on an amount of displacement of the probe. However, the present disclosure is not limited thereto, and other probes with various shapes, such as a probe having a hemispherical tip and a probe having an elliptical columnar tip, may be used. Alternatively, the hardness testers that measure the hardness of the target to be measured by various other methods may be used. In this case, any other means that can acquire a change over time in the physical quantity corresponding to the amount of displacement of the probe may be used. Regarding various values on the microhardness tester, the diameter of the probe may be set to a value other than <NUM> [mm], the descending speed of the probe may be set to a value other than <NUM> [mm/s], and the load may be set to a value that is not between <NUM> and <NUM> [Nm].

Moreover, in the above-described embodiment, the image forming apparatus <NUM> includes four developing units <NUM> (<FIG>). However, the present disclosure is not limited thereto. For example, the image forming apparatus <NUM> may include one to three or five or more developing units <NUM>.

Furthermore, in the above-described embodiment, the present disclosure is applied to the image forming apparatus <NUM>, which is a single-function SFP. However, the present disclosure is not limited thereto, and may be applied to image forming apparatuses with various other functions, such as a Multi Function Peripheral (MFP) that has the functions of a copier and a facsimile machine, for example.

The present disclosure is not limited to the above-described embodiment and modifications. That is, the present disclosure can also be applied to any embodiments obtained by appropriately combining some or all of the respective embodiments and modifications described above, as well as any embodiment obtained by extracting parts of the above embodiment and modifications therefrom.

Furthermore, in the above-described embodiment, the heating belt <NUM> as the annular belt and the pressurizing section <NUM> as the facing member constitute the fixing unit <NUM> as the fixing device. However, the present disclosure is not limited thereto, and a facing member and an annular belt, which can have various other configurations, may constitute the fixing device.

The present disclosure can be used, for example, when a toner image formed on a medium by an electrophotographic method is fixed to the medium by a fixing unit.

Claim 1:
A fixing device (<NUM>) comprising:
an annular belt (<NUM>) having an outer circumferential surface (<NUM>); and
a facing member (<NUM>) that faces the outer circumferential surface (<NUM>) of the annular belt (<NUM>) to form a nip region (N) between the annular belt (<NUM>) and the facing member (<NUM>),
characterised in that
the annular belt (<NUM>) is configured so that in measurement of a hardness of the outer circumferential surface (<NUM>) using a hardness tester "Micro Rubber Hardness Tester MD-1capa" manufactured by Kobunshi Keiki Co., Ltd., in which a cylindrical probe having a diameter of <NUM> [mm] of the hardness tester is pushed into the outer circumferential surface (<NUM>) with a load of <NUM> to <NUM> [Nm] at a speed of <NUM> [mm/s] and the hardness of the outer circumferential surface (<NUM>) is measured based on an amount of the displacement of the probe,
assuming that a first hardness value (A) represents a measured value at a time when a measurement time corresponding to a time required for a predetermined point on the outer circumferential surface (<NUM>) to pass through the nip region (N) has elapsed after start of the hardness measurement, and that a second hardness value (B) represents a measured value at a time when the measured value by the hardness tester is saturated,
a ratio (A/B) of the first hardness value (A) to the second hardness value (B) is <NUM> or more and <NUM> or less.