JP7552402B2 - Fixing device and image forming apparatus - Google Patents

Description

This disclosure relates to a fixing device and an image forming device.

JP 2019-184759 A (Patent Document 1) discloses a fixing device using a pad fixing method. The fixing device includes a fixing belt, a fixing pad including an opposing surface that faces the fixing belt, and a pressure roller that is disposed opposite the fixing pad via the fixing belt and forms a fixing nip with the fixing pad. The opposing surface includes a curved surface portion that is concavely curved so as to approach the center of the pressure roller as it moves downstream in the transport direction of the recording medium, and an extension portion that is provided downstream of the curved surface portion in the transport direction of the recording medium and moves away from the center of the pressure roller as it moves downstream in the transport direction.

JP 2019-184759 A

The fixing pad described in the above document has a convex shape toward the pressure roller downstream of the fixing nip in the transport direction of the recording medium. By positioning the position where the pressure of the fixing nip is highest downstream in the transport direction, the efficiency of melting the toner is improved.

However, the convex parts of the fixing pad can cause the fixing belt to be less able to follow the unevenness of the recording medium's surface, resulting in insufficient melting of the toner. In the fixing nip, particularly near the entrance to the fixing nip, if unfixed toner moves relative to the recording medium due to the sliding resistance between the fixing pad and the fixing belt, the base of the recording medium becomes exposed, resulting in a decrease in image density.

This disclosure proposes a fixing device and an image forming device that can improve image quality.

In accordance with the present disclosure, a fixing device is proposed that fixes a toner image formed on a recording medium to the recording medium. The fixing device includes a fixing belt, a heating member that heats the fixing belt, a first pressure member that contacts the fixing belt, and a second pressure member that presses the first pressure member through the fixing belt to form a fixing nip between the fixing belt and the fixing belt. The fixing device is configured so that the time from the entrance of the fixing nip until the surface pressure of the fixing nip reaches the necessary unevenness following pressure, which is the surface pressure required for the fixing belt to deform and contact the recesses on the surface of the recording medium, is a first time, and the time from the entrance of the fixing nip until the temperature of the toner passing through the fixing nip reaches a temperature required to prevent relative movement of the toner with respect to the recording medium due to the sliding resistance between the fixing belt and the first pressure member is a second time that is shorter than the first time.

The fixing device may be configured so that the time it takes for the temperature of the toner passing through the fixing nip from the entrance of the fixing nip to reach a temperature required for the toner to be sufficiently fixed to the recording medium is a third time that is longer than the first time.

The fixing device may be configured to maintain the surface pressure of the fixing nip at a point when a third time has elapsed from the entrance of the fixing nip, higher than the pressure required to follow the unevenness.

In the fixing device described above, the temperature required to prevent relative movement of the toner with respect to the recording medium may be the temperature at which the shape of the toner begins to change.

In the fixing device, the toner contains a release agent, the release agent contains wax, and the temperature required to prevent relative movement of the toner with respect to the recording medium may be the melting point of the wax.

In the above fixing device, the first pressure member may have a flat surface in a range corresponding to the time from the entrance of the fixing nip until the temperature of the toner passing through the fixing nip reaches the temperature required for sufficient fixing on the recording medium.

In accordance with the present disclosure, an image forming apparatus is proposed that includes an image forming section that forms a toner image on a recording medium, and a fixing device according to any of the above aspects that fixes the toner image to the recording medium.

The fixing device and image forming device disclosed herein can improve image quality.

1 is a diagram illustrating an overall configuration of an image forming apparatus according to an embodiment. 2 is a schematic diagram illustrating a fixing device of the image forming apparatus illustrated in FIG. 1 . FIG. 4 is an enlarged cross-sectional view showing the cross-sectional shape of the fixing pad. 6 is a graph showing pressure distribution in the fixing nip in the paper transport direction. 6 is a graph showing a change in temperature in the fixing nip in the paper transport direction. FIG. 4 is a schematic diagram showing the state of toner at a nip entrance. FIG. 4 is a schematic diagram showing the state of the toner when the toner wax melting point is reached. 11A and 11B are schematic diagrams showing the state of toner when the required pressure reaching time has elapsed. FIG. 4 is a schematic diagram showing the state of a toner when the toner resin reaches its softening point; 11A and 11B are schematic diagrams showing the state of toner when the required pressure holding time has elapsed.

Below, an embodiment of a fixing device and an image forming device according to the present disclosure will be described with reference to the drawings. In the following description, the same parts and components are given the same reference numerals. Their names and functions are also the same. Therefore, detailed descriptions thereof will not be repeated.

[Configuration of image forming apparatus 100]
1 is a diagram that shows a schematic overall configuration of an image forming apparatus 100 according to an embodiment. In the following, the image forming apparatus 100 will be described as a color printer, but the image forming apparatus 100 is not limited to a color printer. For example, the image forming apparatus 100 may be a monochrome printer, a fax machine, or a multi-functional peripheral (MFP) that has the functions of a scanner, printer, and copier.

The image forming apparatus 100 includes image forming units 1Y, 1M, 1C, and 1K, an intermediate transfer belt 30, a primary transfer roller 31, a secondary transfer roller 33, a cassette 37, a driven roller 38, a drive roller 39, a timing roller 40, a fixing device 50, a control device 101, and a housing 102.

The housing 102 defines the outer shell of the image forming device 100. Inside the housing 102, there are housed the image forming units 1Y, 1M, 1C, and 1K, the intermediate transfer belt 30, the primary transfer roller 31, the secondary transfer roller 33, the cassette 37, the driven roller 38, the drive roller 39, the timing roller 40, the fixing device 50, and the control device 101.

The image forming unit is made up of image forming units 1Y, 1M, 1C, and 1K, intermediate transfer belt 30, primary transfer roller 31, secondary transfer roller 33, cassette 37, driven roller 38, drive roller 39, and timing roller 40. This image forming unit forms a toner image on paper S, which serves as a recording medium, transported along a transport path 41, which will be described later.

Image forming units 1Y, 1M, 1C, and 1K are arranged in order along intermediate transfer belt 30. Image forming unit 1Y receives toner from toner bottle 15Y and forms a yellow (Y) toner image. Image forming unit 1M receives toner from toner bottle 15M and forms a magenta (M) toner image. Image forming unit 1C receives toner from toner bottle 15C and forms a cyan (C) toner image. Image forming unit 1K receives toner from toner bottle 15K and forms a black (BK) toner image.

Image forming units 1Y, 1M, 1C, and 1K are arranged along intermediate transfer belt 30 in the order of the rotation direction of intermediate transfer belt 30. Image forming units 1Y, 1M, 1C, and 1K each include a photoconductor 10, a charging device 11, an exposure device 12, a developing device 13, and a cleaning device 17.

The charging device 11 uniformly charges the surface of the photoreceptor 10. The exposure device 12 irradiates the photoreceptor 10 with laser light in response to a control signal from the control device 101, exposing the surface of the photoreceptor 10 according to the input image pattern. As a result, an electrostatic latent image corresponding to the input image is formed on the photoreceptor 10.

The developing device 13 applies a developing bias to the developing roller 14 while rotating the developing roller 14, causing toner to adhere to the surface of the developing roller 14. This causes the toner to be transferred from the developing roller 14 to the photoreceptor 10, and a toner image corresponding to the electrostatic latent image is developed on the surface of the photoreceptor 10.

The photoconductor 10 and the intermediate transfer belt 30 are in contact with each other at the portion where the primary transfer roller 31 is provided. The primary transfer roller 31 has a roller shape and is configured to be rotatable. A transfer voltage of the opposite polarity to that of the toner image is applied to the primary transfer roller 31, so that the toner image is transferred from the photoconductor 10 to the intermediate transfer belt 30. A yellow (Y) toner image, a magenta (M) toner image, a cyan (C) toner image, and a black (BK) toner image are transferred from the photoconductor 10 to the intermediate transfer belt 30 in superimposed order. As a result, a color toner image is formed on the intermediate transfer belt 30.

The toner contains a resin and a colorant dispersed in the resin. The resin may be any conventionally known material without any particular limitation, and examples of the resin include polyester resin, acrylic resin, styrene-acrylic copolymer resin, urethane resin, vinyl chloride resin, and vinyl acetate resin. In addition to the resin and colorant, the toner contains a release agent. The release agent includes wax. The wax may be any conventionally known material without any particular limitation, and examples of the wax include synthetic ester wax.

The intermediate transfer belt 30 is stretched over a driven roller 38 and a drive roller 39. The drive roller 39 is driven to rotate by, for example, a motor (not shown). The intermediate transfer belt 30 and the driven roller 38 rotate in conjunction with the drive roller 39. This causes the toner image on the intermediate transfer belt 30 to be transported to the secondary transfer roller 33.

The cleaning device 17 is pressed against the photoreceptor 10. The cleaning device 17 collects the toner remaining on the surface of the photoreceptor 10 after the toner image is transferred.

Paper S is set in cassette 37 at the bottom of housing 102. Paper S is an example of a recording medium used to record an image formed in image forming device 100. The recording medium may be a paper recording medium such as plain paper, photographic paper, cardboard, or an envelope, or a resin recording medium such as OHP (overhead projector) paper. Paper S is sent one sheet at a time from cassette 37 by timing roller 40 along transport path 41 to secondary transfer roller 33.

The secondary transfer roller 33 has a roller shape and is configured to be rotatable. The secondary transfer roller 33 applies a transfer voltage of the opposite polarity to the toner image to the paper S being transported. As a result, the toner image is attracted from the intermediate transfer belt 30 to the secondary transfer roller 33, and the toner image on the intermediate transfer belt 30 is transferred to the paper S. The primary transfer roller 31, the intermediate transfer belt 30, and the secondary transfer roller 33 correspond to a transfer section that transfers the toner image from the photoconductor 10 to the paper S.

The timing of transporting the paper S to the secondary transfer roller 33 is adjusted by the timing roller 40 to match the position of the toner image on the intermediate transfer belt 30. The timing roller 40 transfers the toner image on the intermediate transfer belt 30 to the appropriate position on the paper S.

The fixing device 50 applies pressure and heat to the paper S passing by it. This causes the toner image on the paper S to be fixed to the paper S. In this way, the fixing device 50 fixes the toner image formed on the paper S transported along the transport path 41 to the paper S. The paper S with the fixed toner image is discharged to the tray 48.

Although the image forming apparatus 100 employs a tandem printing method in the above description, the printing method of the image forming apparatus 100 is not limited to the tandem method. The arrangement of each component in the image forming apparatus 100 may be changed as appropriate according to the printing method employed. The printing method of the image forming apparatus 100 may be a rotary method or a direct transfer method. In the case of the rotary method, the image forming apparatus 100 is composed of one photoconductor 10 and multiple developing devices 13 configured to be rotatable on the same axis. During printing, the image forming apparatus 100 guides each developing device 13 to the photoconductor 10 in sequence to develop a toner image of each color. In the case of the direct transfer method, the image forming apparatus 100 transfers the toner image formed on the photoconductor 10 directly to the paper S.

[Configuration of Fixing Device 50]
Next, a detailed description will be given of the structure of the fixing device 50. Fig. 2 is a schematic diagram showing the fixing device 50 of the image forming apparatus 100 shown in Fig. 1.

As shown in FIG. 2, the fixing device 50 mainly includes a pressure roller 51, an endless fixing belt 52, a heating roller 53 having a fixing heater 54, and a fixing pad 56.

The pressure roller 51 is disposed outside the fixing belt 52. The pressure roller 51 faces the fixing pad 56. The pressure roller 51 presses the fixing pad 56 by sandwiching the fixing belt 52 between them using the biasing force of a biasing member (not shown). This causes the fixing belt 52 to be pressed against the fixing pad 56. The pressure roller 51 presses the fixing pad 56 through the fixing belt 52.

When the pressure roller 51 is compressed and deformed to conform to the shape of the fixing pad 56, a fixing nip 57, which is a contact area between the pressure roller 51 and the fixing belt 52, is formed between the pressure roller 51 and the fixing belt 52. The fixing pad 56 corresponds to the first pressure member in the embodiment. The pressure roller 51 corresponds to the second pressure member in the embodiment.

The pressure roller 51 is composed of, for example, a core metal, a surface layer, and a release layer. The core metal is made of metal such as aluminum or steel, and has a hollow cylindrical shape. The surface layer covers the outer circumferential surface of the core metal. The surface layer is an elastic layer such as silicone rubber. The release layer is provided so as to cover the surface layer. The release layer is composed of, for example, PFA (tetrafluoroethylene-perfluoroalkylvinylether copolymer) or PTFE (polytetrafluoroethylene).

The pressure roller 51 rotates in the direction of the arrow AR1 shown in FIG. 2 due to the torque from the drive motor 69. As the pressure roller 51 rotates, the fixing belt 52 rotates in the direction of the arrow AR3 shown in FIG. 2. As the pressure roller 51 rotates, the fixing belt 52 rotates in the opposite direction to the rotation direction of the pressure roller 51. As the fixing belt 52 rotates, the heating roller 53 rotates in the direction of the arrow AR4 shown in FIG. 2.

The pressure roller 51 transports the paper S by rotating the fixing belt 52. The fixing pad 56 and pressure roller 51 clamp the paper S carrying unfixed toner T in the fixing nip 57 and heat and pressurize the paper S while transporting it in a specified transport direction. As the toner T is heated and pressurized while the paper S passes through the fixing nip 57, the toner T formed on the paper S melts and the image of the toner T is fixed to the paper S.

The length of the pressure roller 51 in the axial direction (in FIG. 2, perpendicular to the paper surface) is longer than the width of the maximum size of paper S that can be used. The same is true for the fixing belt 52, the heating roller 53, and the fixing pad 56. The transport direction of the paper S along the transport path 41 is indicated by the arrow AR2 in FIG. 2. The width of the paper S refers to the dimension of the paper S in a direction perpendicular to the transport direction and perpendicular to the thickness direction of the paper S (in FIG. 2, perpendicular to the paper surface).

The fixing belt 52 is formed in an endless shape. A fixing pad 56, a heating roller 53, and a belt guide member 58 are arranged inside the rotation path of the fixing belt 52. The fixing belt 52 is rotatably wrapped around the fixing pad 56, the heating roller 53, and the belt guide member 58.

The fixing belt 52 has a base layer, an elastic layer covering the base layer, and a release layer covering the elastic layer. The base layer may be made of a metal such as nickel, stainless steel (SUS), or copper, or a resin such as polyimide, polyamide, or polyimideamide. The elastic layer may be made of a material with high heat resistance such as silicone rubber or fluororubber. The release layer may be made of a material with releasability, such as a fluororesin or a fluorine-based coating.

The heating roller 53 is disposed inside the fixing belt 52. The heating roller 53 is disposed at a position away from the fixing pad 56. The heating roller 53 has a core made of a metal material such as aluminum or SUS. The core has a hollow cylindrical shape. The outer circumferential surface of the core may be coated with PTFE or the like to prevent the outer circumferential surface from being scratched by foreign matter or the like. The outer circumferential surface of the core may be coated with a heat-resistant release layer.

A fixing heater 54 is disposed inside the core metal as a heating member that heats the fixing belt 52. The heating roller 53 has the fixing heater 54 built in. The fixing heater 54 heats the core metal from the inside. The heat generated by the fixing heater 54 is transmitted to the fixing belt 52 via the heating roller 53, and is further transmitted to the paper S passing through the fixing nip 57 via the fixing belt 52.

The fixing heater 54 is, for example, a halogen heater. When a halogen heater is used as the fixing heater 54, the inner surface of the core metal may be black. The heating member that heats the fixing belt 52 may be an induction heating heater or a carbon heater other than a halogen heater, and the heating roller 53 and the fixing belt 52 may be used as resistance heating elements to generate heat.

The fixing pad 56 is disposed inside the fixing belt 52. The fixing pad 56 is disposed so as to face the pressure roller 51 with the fixing belt 52 sandwiched therebetween. The fixing pad 56 is formed of a heat-resistant resin such as polyphenylene sulfide, polyimide, or liquid crystal polymer. The fixing pad 56 is non-rotatable, and the rotating fixing belt 52 slides against the surface of the fixing pad 56.

Figure 3 is an enlarged cross-sectional view showing the cross-sectional shape of the fixing pad 56. As shown in Figures 2 and 3, the fixing pad 56 has a contact surface 80 that contacts the inner circumferential surface of the fixing belt 52, and a support surface 81 that abuts against the support member 55. The contact surface 80 faces the inner circumferential surface of the fixing belt 52. The pressure roller 51 elastically deforms to follow the surface shape of the contact surface 80.

The contact surface 80 has, from the upstream side to the downstream side in the transport direction of the paper S, a convex curved third surface 74, a flat first surface 71, and a flat second surface 72. The term "flat" here means roughly flat, and also includes a surface that has a small curvature and is slightly curved. If the curvature is such that no wrinkles are generated in the envelope when the envelope is passed through the fixing device 50, it may be considered to be flat.

The imaginary plane 95 extending from the first surface 71 and the imaginary plane 96 extending from the second surface 72 intersect at an angle so that the contact surface 80 protrudes toward the pressure roller 51. The first surface 71 and the second surface 72 intersect with each other. The contact surface 80 as a whole is formed to protrude toward the pressure roller 51. The convex portion 73 where the first surface 71 and the second surface 72 intersect is the protruding end where the contact surface 80 protrudes the most.

The convex portion 73 is provided between the nip entrance 57A, which is the upstream start end of the fixing nip 57 in the transport direction of the paper S, and the nip exit 57B, which is the downstream end end of the fixing nip 57 in the transport direction. The nip entrance 57A is the start end where pressure begins to be applied to the paper S and toner T passing through the fixing nip 57. The nip exit 57B is the end end where pressure ends to the paper S and toner T passing through the fixing nip 57.

The first surface 71 includes a flat surface between the nip entrance 57A and the convex portion 73. The second surface 72 includes a flat surface between the convex portion 73 and the nip exit 57B. In the conveying direction, the first surface 71 is located upstream of the convex portion 73, and the second surface 72 is located downstream of the convex portion 73. The convex portion 73 constitutes the downstream end of the first surface 71 in the conveying direction. The convex portion 73 constitutes the upstream end of the second surface 72 in the conveying direction.

The convex portion 73 is provided in the rear half of the fixing nip 57. The length of the first surface 71 in the transport direction of the paper S is longer than the length of the second surface 72 in the transport direction of the paper S. The convex portion 73 is disposed on the exit side of the fixing nip 57. The length from the nip entrance 57A to the convex portion 73 in the transport direction of the paper S is longer than the length from the convex portion 73 to the nip exit 57B. The first surface 71 gradually protrudes toward the pressure roller 51 as it moves downstream in the transport direction of the paper S. The second surface 72 moves away from the center of the pressure roller 51 as it moves downstream in the transport direction of the paper S. The pressure in the fixing nip 57 increases as it moves from the nip entrance 57A to the convex portion 73 and decreases as it moves from the convex portion 73 to the nip exit 57B.

The third surface 74 is a curved surface extending from the upstream end 91 of the first surface 71 in a direction away from the transport path 41 beyond an imaginary plane 95 that is an extension of the flat first surface 71 upstream in the transport direction of the paper S. The fixing pad 56 also has a curved surface on the downstream side in the transport direction of the paper S from the downstream end 98 of the second surface 72.

Returning to FIG. 2, the fixing pad 56 is supported by the support member 55 on the support surface 81. The support member 55 is made of a metal, for example, aluminum. When viewed along the width direction of the fixing belt 52 (perpendicular to the paper surface in FIG. 2), the support member 55 has an angular U-shape. The support member 55 is arranged so that the tip side of the U-shape faces away from the pressure roller 51. The fixing pad 56 is supported by the bottom of the U-shape of the support member 55 and the arm of the U-shape on the upstream side of the rotation direction of the fixing belt 52. The belt guide member 58 is supported by the arm of the U-shape on the downstream side of the rotation direction of the fixing belt 52.

The support member 55 suppresses deformation of the fixing pad 56 that receives pressure from the pressure roller 51, thereby enabling a fixing nip 57 of uniform width to be obtained in the axial direction of the pressure roller 51 (in FIG. 2, the direction perpendicular to the paper surface).

[Setting of Operation of Fixing Device 50]
The operation settings for obtaining good image quality for the fixing device 50 having the above configuration will be described. Fig. 4 is a graph showing the pressure distribution in the fixing nip 57 in the transport direction of the paper S. The horizontal axis of Fig. 4 indicates the position in the fixing nip 57 (from the nip entrance 57A to the nip exit 57B) in the transport direction of the paper S. The nip surface pressure (average in the axial direction) shown on the vertical axis of Fig. 4 indicates the average result of the pressure applied per unit area in the fixing nip 57 receiving the load of the pressure roller 51 pressing the fixing pad 56, in the axial direction of the pressure roller 51.

The nip surface pressure can be measured by inserting a sheet-shaped pressure sensor (e.g., a tactile sensor manufactured by Nitta Corporation) into the fixing nip 57 while the pressure roller 51 is applying a load. The time it takes for the paper S to pass through the fixing nip 57 can be calculated from the transport speed of the paper S and the measurement results of the nip surface pressure.

The dashed line parallel to the horizontal axis in the graph of FIG. 4 indicates the surface pressure (hereinafter referred to as the required pressure P for unevenness tracking) required for the elastic layer on the surface of the fixing belt 52 to deform and contact the recesses on the surface of the paper S due to the load of the pressure roller 51 pressing against the fixing pad 56 in the fixing nip 57. Because the paper S is composed of multiple long fibers intertwined, concave voids (recesses) are formed between the intertwined fibers. Even ordinary plain paper has unevenness on its surface. Embossed paper has larger unevenness than plain paper.

The unevenness-following pressure P is the pressure required for the surface of the fixing belt 52 to deform to correspond to the maximum unevenness height on the surface of each type of paper S that can be passed through. The unevenness-following pressure P differs when it is assumed that only plain paper will be passed through the fixing device 50 from when it is assumed that both plain paper and embossed paper will be passed through, with the unevenness-following pressure P being higher in the latter case. The unevenness-following pressure P can be determined by computer simulation or actual measurement from the hardness of the elastic layer on the surface of the fixing belt 52, the unevenness height on the surface of the paper S, and the period of the unevenness on the surface of the paper S in the transport direction.

As shown in FIG. 4, the nip surface pressure increases from the nip entrance (point A in FIG. 4, which corresponds to nip entrance 57A shown in FIG. 3) in the transport direction of the paper S, and increases until it becomes greater than the necessary pressure P for unevenness tracking. Point B in FIG. 4 indicates the position in the transport direction of the upstream end of the region where the nip surface pressure is reliably maintained at or above the necessary pressure P for unevenness tracking, even when measurement variation is taken into account. The time it takes for the paper S transported through the fixing nip 57 to reach point B from point A (nip entrance) is referred to as the necessary pressure arrival time t1. The necessary pressure arrival time t1 corresponds to the first time in the embodiment.

Point C in FIG. 4 indicates the position in the conveying direction of the downstream end of the region where the nip surface pressure is reliably maintained at or above the required unevenness-following pressure P, even when measurement variation is taken into account. The time it takes for the paper S conveyed through the fixing nip 57 to reach point C from point B is referred to as the required pressure holding time t2.

Between points B and C, there is a point where the nip surface pressure is at its maximum value. As shown in FIG. 4, the nip surface pressure increases suddenly from the nip entrance to point B, increases gradually from point B to the maximum pressure, decreases gradually after reaching the maximum pressure to point C, and then decreases suddenly from point C.

The required pressure reaching time t1 and the required pressure holding time t2 can be calculated from the shape and dimensions of the fixing pad 56, the hardness of the outer peripheral surface of the pressure roller 51, the dimensions of the pressure roller 51, and the load with which the pressure roller 51 presses against the fixing pad 56.

Figure 5 is a graph showing the temperature change in the fixing nip 57 in the transport direction of the paper S. As with Figure 4, the horizontal axis of Figure 5 indicates the position in the fixing nip 57 (from the nip entrance 57A to the nip exit 57B) in the transport direction of the paper S. The vertical axis of Figure 5 indicates the temperature at each position in the fixing nip 57. In Figure 5, the temperature of the surface of the fixing belt 52 is shown by a solid line, the temperature of the toner T on the paper S is shown by a dashed line, and the temperature of the surface of the paper S is shown by a dashed line.

The temperature of the toner T and the temperature of the surface of the paper S while the paper S passes through the fixing nip 57 can be determined by computer simulation. The temperature of the toner T and the temperature of the surface of the paper S can be determined from the physical properties and structure of the fixing device 50, such as the control method of the fixing heater 54, the amount of heat transferred from the fixing heater 54 to the heating roller 53, and the amount of heat transferred from the heating roller 53 to the fixing belt 52, as well as the physical properties of the toner T. Alternatively, the temperature of the toner T and the temperature of the surface of the paper S can be actually measured by passing the paper S, to which a predetermined temperature sensor, such as a thermocouple, is fixed on the surface that contacts the fixing belt 52, through the fixing nip 57.

Of the two dashed lines parallel to the horizontal axis in the graph of FIG. 5, the line closer to the horizontal axis indicates the melting point of the wax contained in the release agent contained in toner T (hereinafter referred to as the toner wax melting point Twm).Of the two dashed lines parallel to the horizontal axis in the graph of FIG. 5, the line farther from the horizontal axis indicates the softening point temperature of the resin that constitutes toner T (hereinafter referred to as the toner resin softening point Tm).

The toner wax melting point Twm is lower than the toner resin softening point Tm, and may be a temperature at which the wax contained in the toner T melts to such an extent that the toner T does not move relative to the paper S due to the shear force caused by the sliding resistance between the fixing belt 52 and the fixing pad 56. If a large shear force acts on the toner T before the toner T is adequately melted, the toner T will move relative to the paper S, and this is intended to prevent this. The temperature of the toner T is used as an index for confirming that the toner T is adequately melted. The temperature of the toner T may be the temperature at the center in the thickness direction of the layer of the toner T formed on the paper S. The temperature of the toner T may be the temperature at the interface between the toner T and the paper S.

The toner resin softening point Tm is set as a condition for the toner T to completely melt. It is considered that the toner T is sufficiently fixed to the paper S if the temperature of the interface between the toner T and the paper S is equal to or higher than the toner resin softening point Tm due to heat transfer from the fixing belt 52 that contacts the surface of the toner T. The temperature of the interface between the toner T and the paper S may be limited to the temperature at the concave portion of the uneven surface of the paper S. The purpose is to prevent the toner T in the concave portion from moving relative to the paper S due to the action of shear force when the toner T present in the concave portion is not completely fixed to the paper S.

The toner wax melting point Twm and the toner resin softening point Tm are determined according to the physical properties of the toner T, such as the types of resin and wax contained in the toner T, the ratio of wax contained in the toner T, and the particle size of the toner T. When the temperature of the toner T is between the toner wax melting point Twm and the toner resin softening point Tm, the force of the pressure roller 51 that fixes the toner T to the paper S is greater than the force that moves the toner T relative to the paper S due to the shear force.

Points A, B, and C in Figure 5 are as described with reference to Figure 4. Point D is the intersection of the curve showing the temperature of toner T with the toner wax melting point Twm, and indicates the position where the temperature of toner T reaches the toner wax melting point Twm. Point E is the intersection of the curve showing the temperature of toner T with the toner resin softening point Tm, and indicates the position where the temperature of toner T reaches the toner resin softening point Tm.

As shown in FIG. 5, the fixing device 50 is set so that the time required for the temperature of the toner T passing through the fixing nip 57 to reach the toner wax melting point Twm is shorter than the required pressure reaching time t1. The time required for the toner T to be transported from point A to point B shown in FIG. 4 and 5 is set longer than the time required for the toner T to be transported from point A to point D shown in FIG. 5. The fixing device 50 is configured so that the time required for the surface pressure to reach the unevenness following required pressure P from the nip entrance 57A is longer than the time required for the temperature of the toner T passing through the fixing nip 57 to reach the temperature required to prevent relative movement of the toner T with respect to the paper S. The time required for the temperature of the toner T passing through the fixing nip 57 to reach the toner wax melting point Twm from the nip entrance 57A corresponds to the second time in the embodiment.

The fixing device 50 is set so that the time until the temperature of the toner T passing through the fixing nip 57 reaches the toner resin softening point Tm is longer than the required pressure reaching time t1. The time from when the toner T reaches the nip entrance 57A until the temperature of the toner T reaches the toner resin softening point Tm can be said to be the time required for the toner T passing through the fixing nip 57 to be sufficiently fixed to the paper S. The time required for the toner T to be transported from point A to point B shown in Figures 4 and 5 is set shorter than the time required for the toner T to be transported from point A to point E shown in Figure 5. The fixing device 50 is configured so that the time from the nip entrance 57A until the surface pressure reaches the unevenness following required pressure P is shorter than the time until the temperature of the toner T passing through the fixing nip 57 reaches the temperature required for sufficient fixing to the paper S. The time from the nip entrance 57A until the temperature of the toner T passing through the fixing nip 57 reaches the toner resin softening point Tm corresponds to the third time in the embodiment.

The fixing device 50 is set so that the sum of the required pressure reaching time t1 and the required pressure holding time t2 is longer than the time required for the temperature of the toner T to reach the toner resin softening point Tm. The time required for the toner T to be transported from point A to point C shown in Figures 4 and 5 is set longer than the time required for the toner T to be transported from point A to point E shown in Figure 5. The fixing device 50 is configured so that the time for maintaining the unevenness following required pressure P from the nip entrance 57A is longer than the time required for the temperature of the surface of the paper S (the image surface on which the image of the toner T is formed) passing through the fixing nip 57 to reach the temperature required for the temperature of the toner T to be sufficiently fixed to the paper S. The fixing device 50 is configured to maintain the surface pressure of the fixing nip 57 higher than the unevenness following required pressure P at the time when the time has elapsed from the nip entrance 57A until the temperature of the toner T passing through the fixing nip 57 reaches the temperature required for the temperature of the toner T to be sufficiently fixed to the paper S.

Referring also to FIG. 3, the contact surface 80 of the fixing pad 56 has a flat first surface 71. The first surface 71 extends parallel to the transport direction of the paper S. The first surface 71 is not curved relative to the transport direction of the paper S. The fixing pad 56 has a flat surface in a range from the nip entrance 57A to the time it takes for the temperature of the toner T to reach the toner resin softening point Tm. The fixing pad 56 has a flat surface in a range from the nip entrance 57A to the time it takes for the temperature of the toner T passing through the fixing nip 57 to reach a temperature required for sufficient fixing to the paper S.

The required pressure holding time t2 can be changed. The position in the fixing nip 57 at the end of the required pressure holding time t2 (corresponding to point C shown in Figs. 4 and 5) may be located downstream of the fixing nip 57 (closer to the nip exit 57B) than the position in the fixing nip 57 where the temperature of the toner T becomes equal to or higher than the toner resin softening point Tm (corresponding to point E shown in Fig. 5). By making the required pressure holding time t2 longer, the toner T in the recesses on the surface of the paper S can be efficiently fixed to the paper S. The required pressure holding time t2 may be changed according to the preference of the user who uses the image forming apparatus 100 and the type of paper passed through the fixing device 50. The required pressure holding time t2 may be set for each model of the image forming apparatus 100 according to the gloss design, paper type compatibility, etc.

[Melting of Toner T in Fixing Nip 57]
Fig. 6 is a schematic diagram showing the state of toner T at the nip entrance. Fig. 6 also shows a schematic diagram of toner T formed on paper S when paper S passes through the entrance of fixing nip 57 (nip entrance 57A shown in Fig. 3, point A shown in Figs. 4 and 5). Arrow AR2 shown in Fig. 6 and Figs. 7 to 10 described below indicates the conveyance direction of paper S described with reference to Fig. 2. Arrow X indicates the direction in which the sliding friction resistance between fixing belt 52 and fixing pad 56 acts on fixing belt 52.

Referring also to FIG. 5, at the nip entrance 57A, the amount of heat transferred from the fixing belt 52 to the toner T is small, and the temperature of the toner T is lower than the toner wax melting point Twm. Therefore, not all of the toner T is melted, and the toner T maintains its particulate shape. At the nip entrance 57A, the load acting on the fixing belt 52 due to the pressure of the pressure roller 51 is also low, and therefore no relative movement of the toner T occurs with respect to the paper S.

Figure 7 is a schematic diagram showing the state of toner T when it reaches the toner wax melting point Twm. Figure 7 also shows a schematic of toner T formed on paper S when paper S passes the position of point D. The downward white arrows shown in Figure 7 and Figures 8 to 10 described below show a schematic of the load acting on fixing belt 52 due to the pressure of pressure roller 51.

As shown in FIG. 7, when the temperature of the toner T reaches the toner wax melting point Twm, the toner T at the interface with the fixing belt 52 is partially melted, but the load caused by the pressure of the pressure roller 51 is low, so the toner T does not deform. When the temperature of the toner T becomes higher than the toner wax melting point Twm, the toner T begins to deform. The toner wax melting point Twm can be said to be the temperature at which the shape of the toner T begins to change.

Figure 8 is a schematic diagram showing the state of the toner T when the required pressure reaching time t1 has elapsed. Figure 8 shows a schematic diagram of the toner T formed on the paper S when the paper S passes the position of point B.

Referring also to FIG. 5, the temperature of the toner T is higher than the toner wax melting point Twm, so the toner T is partially melted. Referring also to FIG. 4, the nip surface pressure is higher than the necessary pressure P for conforming to the irregularities, so the nip surface pressure required for the deformation of the toner T is acting. Due to the load caused by the pressing action of the pressure roller 51, a portion of the toner T on the interface side with the fixing belt 52 is deformed.

The load with which the pressure roller 51 presses against the fixing pad 56 via the fixing belt 52 increases, increasing the sliding resistance between the fixing belt 52 and the fixing pad 56. This causes a frictional force to act on the toner T from the fixing belt 52 in the direction of the arrow X. At this time, the molten toner T at the interface with the fixing belt 52 deforms, so that the toner T at the interface with the paper S does not move relative to the paper S.

Figure 9 is a schematic diagram showing the state of toner T when it reaches the toner resin softening point Tm. Figure 9 also shows the toner T formed on paper S when paper S passes the position of point E.

When the temperature of the toner T reaches the toner resin softening point Tm, the toner T at the interface with the fixing belt 52 melts and forms an integrated layer structure, as shown in FIG. 9. As explained with reference to FIG. 8, the deformation of the molten toner T at the interface with the fixing belt 52 absorbs the frictional force in the direction of the arrow X, so that the toner T at the interface with the paper S does not move relative to the paper S.

Figure 10 is a schematic diagram showing the state of the toner T when the required pressure holding time t2 has elapsed. Figure 9 shows a schematic diagram of the toner T formed on the paper S when the paper S passes the position of point C.

Referring also to FIG. 5, when the temperature of the surface of the paper S reaches the toner resin softening point Tm, the entire toner T melts. The melted toner T deforms to fit the uneven shape of the surface of the paper S. This completes the fixing of the toner T image to the paper S.

[Action and Effects]
Although some of the description overlaps with the above description, the characteristic configuration and effects of this embodiment can be summarized as follows.

5, the fixing device 50 is configured so that the time taken for the temperature of the toner T passing through the fixing nip 57 from the nip entrance 57A to reach a temperature required to prevent relative movement of the toner T with respect to the paper S due to the sliding resistance between the fixing belt 52 and the fixing pad 56 is shorter than the required pressure arrival time t1. More specifically, the time taken for the temperature of the toner T to reach the temperature at which the shape of the toner T begins to change, that is, the toner wax melting point Twm, is set to be shorter than the required pressure arrival time t1.

By making the temperature of the toner T reach the toner wax melting point Twm before the nip surface pressure reaches the unevenness-following pressure P, as shown in Figures 7 and 8, the wax can be melted before the unfixed toner T on the surface of the paper S moves relatively downstream in the transport direction due to shear force. As a result of preventing the unfixed toner T from shifting from its original position, the decrease in the coverage rate of the toner T on the entire paper S is prevented, and the decrease in image density can be efficiently prevented. Therefore, the fixing device 50 and image forming apparatus 100 of the embodiment can improve image quality.

As shown in FIG. 5, the fixing device 50 is configured so that the time it takes for the temperature of the toner T passing through the fixing nip 57 from the nip entrance 57A to reach the temperature required for the toner T to be sufficiently fixed to the paper S, specifically the toner resin softening point Tm, is longer than the required pressure reaching time t1.

After the nip surface pressure reaches the pressure P required to follow the unevenness, the temperature of the toner T is allowed to reach the toner resin softening point Tm, so that the entire toner T can be melted, as shown in Figures 9 and 10. Sufficient pressure and heat can be applied to the toner T present in the recesses on the surface of the paper S to ensure that the toner T is fixed to the recesses, thereby preventing a decrease in image density and improving image quality.

As shown in Figures 4 and 5, the fixing device 50 is configured to maintain the nip surface pressure at the time when the temperature of the toner T reaches the toner resin softening point Tm higher than the required pressure P for following the unevenness.

Until the temperature of the toner T reaches the toner resin softening point Tm, the nip surface pressure higher than the required pressure P for conforming to the irregularities is maintained, so that the molten toner T can be deformed to conform to the irregularities on the surface of the paper S, as shown in Figures 9 and 10, and the toner T can be reliably fixed to the recesses.

As shown in FIG. 3, the fixing pad 56 has a flat surface in the range from the nip entrance 57A to the area corresponding to the time it takes for the temperature of the toner T to reach the toner resin softening point Tm.

By flattening the shape of the fixing pad 56 that comes into contact with the fixing belt 52 before the toner T is sufficiently melted, it is possible to effectively suppress the shear force caused by the sliding resistance between the fixing belt 52 and the fixing pad 56.

The embodiments disclosed herein are illustrative in all respects and should not be considered limiting. The scope of the present invention is indicated by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

Reference Signs List 50 Fixing device, 51 Pressure roller, 52 Fixing belt, 53 Heating roller, 54 Fixing heater, 55 Support member, 56 Fixing pad, 57 Fixing nip, 57A Nip inlet, 57B Nip outlet, 58 Belt guide member, 69 Drive motor, 71 First surface, 72 Second surface, 73 Convex portion, 74 Third surface, 80 Contact surface, 81 Support surface, 91 Upstream end, 95, 96 Virtual plane , 98 Downstream end, 100 Image forming apparatus.

Claims (4)

A fixing device that fixes a toner image formed on a recording medium to the recording medium, comprising:
A fixing belt;
a heating member for heating the fixing belt;
a first pressure member that contacts the fixing belt;
a second pressure member that presses the first pressure member through the fixing belt to form a fixing nip between the fixing belt and the second pressure member,
the toner includes a resin, a colorant dispersed in the resin, and a release agent, the release agent including a wax;
the first pressure member has a flat surface in a range corresponding to a time from an entrance of the fixing nip until a temperature of the toner passing through the fixing nip reaches a softening point temperature of the resin constituting the toner,
a first time is a time from an entrance of the fixing nip until the surface pressure of the fixing nip reaches a required unevenness-following pressure, which is a surface pressure necessary for the fixing belt to deform and contact the recesses on the surface of the recording medium, and a second time is a time from the entrance of the fixing nip until the temperature of the toner passing through the fixing nip reaches a melting point of the wax contained in the release agent , the second time being shorter than the first time. 2. The fixing device according to claim 1, wherein a time taken for the temperature of the toner passing through the fixing nip from an entrance of the fixing nip to reach the softening point temperature is a third time longer than the first time. The fixing device according to claim 2, configured to maintain the surface pressure of the fixing nip at the time when the third time has elapsed from the entrance of the fixing nip higher than the required pressure for following the unevenness. an image forming unit that forms a toner image on a recording medium;
An image forming apparatus comprising: the fixing device according to claim 1 , which fixes the toner image onto the recording medium.

Images