JP2010020129A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing device having satisfactory fixability, separability, etc., despite its simple configuration and regardless of a sheet type and to provide an image forming apparatus. <P>SOLUTION: A fixing section 20 in the image forming apparatus includes: a pressure contact member 41 which is disposed in contact with the back of a pressure belt 22 and firmly presses the pressure belt 22 toward a heat roller 21; a sheet information detecting section 55 which is located upstream of the fixing section 20 and detects the thickness of a conveyed sheet; a moving member 45 which moves the pressure contact member 41 in the direction of sheet conveyance; and a control section 56 which causes the moving member 45 to adjust the position of the pressure contact member 41 based on the sheet thickness detected by the sheet information detecting section 55. The pressure contact member 41 has a rigid pad 42 and an elastic pad 43. If a sheet is thin, the control section 56 shortens a nip portion formed by the elastic pad 43 and increases the amount at which the rigid pad 42 is pressed against the sheet. If a sheet is thick, it lengthens the nip portion and decreases the amount at which the rigid pad 42 is pressed against the sheet. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention has a fixing device that fixes a toner image on a sheet by passing a sheet carrying a toner image between nips of at least one pair of fixing members, and the fixing device. The present invention relates to an image forming apparatus. More specifically, the present invention relates to a fixing device and an image forming apparatus in which one of a pair of fixing members has a cylindrical belt member.

  2. Description of the Related Art Conventionally, an electrophotographic image forming apparatus has a fixing device for fixing a toner image on a sheet. As a fixing device, for example, there is a fixing device that has a pair of rotating members and heats and presses a sheet passing through a nip formed therebetween. In such a fixing device, it has been found that there is a considerable difference in fixing property depending on the material of the sheet to be passed.

  For example, the fixing property is good for a thin sheet, but is often slightly inferior for a cardboard. On the other hand, as for the separation between the rotating member of the fixing device and the sheet, the thinner the sheet, the lower the separation. Therefore, a fixing device that detects the thickness of a sheet before entering the fixing device and adjusts the nip width and pressure contact pressure according to the thickness of the sheet has been proposed (for example, Patent Document 1 and Patent Document 2). reference).

  In the fixing device described in Patent Document 1, the pressing member disposed on the inner peripheral side of the endless belt-like rotating member is moved according to the thickness of the sheet. The pressing member has a long elastic body supported by the holder, and the size of the nip can be changed by swinging the holder about two swing fulcrums. For this purpose, there are two drive means, each of which is controlled.

Further, Patent Document 2 discloses a fixing device having a pressure plate that is slidably disposed on the inner peripheral side of the pressure belt. In the apparatus of this document, an L-shaped lever is rotated by rotating a cam according to the type of sheet, and the pressure plate is swung. Thus, the distance from the nip start position until the fixing load is applied by the pressure plate can be changed.
JP-A-10-228200 JP-A-11-282295

  However, any of the conventional fixing devices described above has a considerably complicated structure. As a result, there is a risk that the size of the apparatus will increase or the cost will increase.

  The present invention has been made to solve the problems of the conventional fixing device described above. That is, an object of the present invention is to provide a fixing device and an image forming apparatus that can realize a good fixing property, separation property and the like with a simple configuration regardless of the type of sheet.

  In order to solve this problem, the fixing device of the present invention includes a pair of fixing members that rotate together and at least one of which is a belt shape, and a sheet carrying a toner image is interposed between the pair of fixing members. Is a fixing device that fixes a toner image on a sheet by passing it through the belt, and is disposed in contact with the back surface of the belt-shaped fixing member, and presses the belt-shaped fixing member toward the other fixing member. Detected by a pressure contact member, a sheet thickness detection member that detects the thickness of the conveyed sheet at a position upstream of the pair of fixing members, a moving member that moves the pressure contact member in the sheet conveyance direction, and a sheet thickness detection member And a controller that adjusts the position of the pressure contact member based on the sheet thickness, and the pressure contact member presses the belt-shaped fixing member against the other fixing member, thereby A first pressure contact member that forms a nip portion having a finite length in the sheet conveyance direction between the receiving members, and is positioned on the downstream side in the sheet conveyance direction with respect to the first pressure contact member; And a second pressing member protruding toward the fixing member. When the sheet thickness is small, the control unit moves the pressing member to a downstream position in the sheet conveyance direction, thereby The length of the part is shortened and the amount of biting of the second pressure contact member into the other fixing member is increased. When the sheet is thick, the pressure contact member is moved to the upstream position in the sheet conveyance direction. , The length of the nip portion is increased and the amount of biting of the second pressure contact member into the other fixing member is decreased.

  According to the fixing device of the present invention, since the belt-shaped fixing member is pressed against the other fixing member from the back surface by the pressing member, a fixing nip portion can be formed between them. Here, since there is a moving member for moving the pressure contact member in the sheet conveying direction, if the other fixing member is curved in the sheet conveying direction, the belt-like fixing member is moved to the other fixing member. The pressure contact state can be adjusted. For example, if the other fixing member is in the shape of a roller, it is most strongly pressed at a position where the sheet conveying direction is the tangential direction, and the pressing force decreases as it moves away.

  Furthermore, since the pressure contact member of the present invention has the first pressure contact member and the second pressure contact member on the downstream side, the length of the nip portion is adjusted according to the position of the first pressure contact member, The outlet pressure is adjusted by the two pressure contact members biting into the other fixing member. When the sheet thickness is thin, the length of the nip portion is shortened and the amount of biting is increased, so that it is possible to have both an appropriate fixing property and a good separation property. On the other hand, when the sheet thickness is thick, the length of the nip portion is increased and the amount of biting is reduced, so that it is possible to have both good fixing properties and appropriate separation properties. Therefore, it is possible to realize good fixing property, separation property and the like regardless of the type of sheet.

Further, in the present invention, it is desirable that the other fixing member has an elastic layer, and the elastic layer is harder than the first pressure contact member and softer than the second pressure contact member.
If it is such, in the location where a 1st press-contact member is press-contacted with respect to an elastic layer, a 1st press-contact member can deform | transform and can form the nip part along an elastic layer. In addition, the second pressure contact member can bite into the elastic layer at a location where the second pressure contact member is pressed against the elastic layer.

Furthermore, in the present invention, it is desirable that the moving member moves the pressure contact member without changing the angle with respect to the sheet conveyance direction.
If it is such, control of the attitude | position of a press-contact member and control of a movement amount are easy. Therefore, the moving member can be configured simply.

  In addition, the present invention includes a pair of fixing members that rotate together with at least one of the image forming portions and at least one of which is a belt-like shape, and a sheet carrying a toner image applied by the image forming portion. An image forming apparatus having a fixing device for fixing a toner image on a sheet by passing between them, the fixing device being arranged in contact with the back surface of a belt-like fixing member, A fixing member that presses the fixing member toward the other fixing member, a sheet thickness detection member that detects the thickness of the conveyed sheet at a position upstream of the pair of fixing members, and the conveying member that conveys the pressing member. A moving member that moves in the direction, and a control unit that causes the moving member to adjust the position of the pressure contact member based on the sheet thickness detected by the sheet thickness detection member. A first pressure contact member that forms a nip portion having a finite length in the sheet conveyance direction between the two fixing members by pressing the fixed fixing member against the other fixing member, and in the sheet conveyance direction more than the first pressure contact member And a second press-contact member that is provided on the downstream side and protrudes toward the other fixing member from the first press-contact member, and when the sheet thickness is thin, the control unit attaches the press-contact member to the sheet. When the sheet thickness is thick, the nip portion is shortened and the amount of biting into the other fixing member is increased. Is moved to an upstream position in the sheet conveying direction, thereby extending the length of the nip portion and reducing the amount of biting of the second pressure contact member into the other fixing member.

  According to the fixing device and the image forming apparatus of the present invention, it is possible to realize good fixing property, separation property and the like with a simple configuration regardless of the type of sheet.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the best mode for embodying the present invention will be described in detail with reference to the accompanying drawings. In this embodiment, the present invention is applied to an electrophotographic image forming apparatus.

  As shown in FIG. 1, the image forming apparatus 1 of the present embodiment has a charging unit 12, an exposure unit 13, a developing unit 14, a transfer unit 15, a separation claw 16, and a cleaner 17 with a photoreceptor 11 as a center. . Further, a sheet tray 19 is provided in the lower part of the figure, and a fixing unit 20 is provided in the upper right part of the figure. Paper is stored in the paper tray 19.

  At the time of image formation, the photoconductor 11 is rotated as indicated by an arrow in FIG. The surface of the photoreceptor 11 is uniformly charged by the charging unit 12, and the exposure unit 13 performs image exposure based on the image data. As a result, an electrostatic latent image is formed on the surface of the photoconductor 11. The electrostatic latent image is developed by the developing unit 14 to form a toner image. The toner image is transferred to the paper conveyed from the paper tray 19 in the transfer unit 15. The sheet carrying the toner image is separated from the photoconductor 11 by the separation claw 16 and conveyed to the fixing unit 20. Further, the toner remaining on the photoreceptor 11 even after passing through the transfer portion 15 is removed by the cleaner 17.

  As shown in FIG. 2, the fixing unit 20 of this embodiment includes a heating roller 21 and a pressure belt 22. The sheet is conveyed upward from below in the figure with the toner carrying surface directed toward the heating roller 21. The toner image is fixed between the heating roller 21 and the pressure belt 22 and further conveyed upward in the figure. A separation claw 23 and a thermistor 24 are provided on the outer periphery of the heating roller 21. Further, a heater 25 is disposed inside the heating roller 21. Thereby, during the fixing process, the surface of the heating roller 21 is kept at an appropriate temperature.

  The heating roller 21 has a cored bar 31 and an elastic layer 32 around it. The core metal 31 is a pipe-shaped member made of metal such as aluminum or iron. The thickness of the cored bar 31 is about 0.1 to 5 mm. However, it should not be too thick to reduce weight and shorten warm-up time. For example, a range of about 0.1 to 1.5 mm is preferable. The elastic layer 32 is formed of a material having both elasticity and heat resistance, such as silicone rubber and fluorine rubber. The thickness of the elastic layer 32 is preferably in the range of 0.05 to 2 mm.

  Further, a release layer may be provided on the outer surface of the elastic layer 32. The release layer is formed by a tube of fluororesin such as PFA (tetrafluoroethylene / perfluoroalkoxyethylene copolymer), PTFE (tetrafluoroethylene), ETFE (tetrafluoroethylene / ethylene copolymer). Good. Alternatively, a silicone resin may be used instead of the fluorine resin. Alternatively, instead of covering the tube, a coating may be used.

  When the release layer is provided, the thickness is preferably about 5 to 100 μm. The contact angle with water is 90 degrees or more, preferably 110 degrees or more. The surface roughness is preferably about 0.01 to 50 μm in terms of Ra (arithmetic mean roughness). Furthermore, it is good also as what has electroconductivity. Examples of fluororesin tubes include PFA350-J, 451HP-J, 951HP Plus manufactured by Mitsui DuPont Fluorochemical Co., Ltd., and the like. The outer diameter of the heating roller 21 is preferably within a range of 10 to 50 mm.

  The pressure belt 22 is a flexible endless belt-like member. The pressure belt 22 has a base material made of a material such as polyimide, polyphenylene sulfide, nickel, iron, or SUS. The thickness of the substrate is preferably about 30 to 300 μm. In addition, it is desirable that the pressure belt 22 of the present embodiment has a size such that the outer diameter when it is cylindrical is in the range of 20 to 100 mm.

  Furthermore, the pressurizing belt 22 may be provided with a release layer made of a fluorine-based tube such as PFA, PTFE, ETFE, or a fluorine-based or silicone-based coating on the outer surface of the substrate. When this release layer is provided, the thickness is desirably about 50 to 150 μm. Further, it may be conductive. Further, the pressure belt 22 may further include an intermediate layer between the base material and the release layer. When the intermediate layer is provided, a material having elasticity and heat resistance such as silicone rubber or fluororubber may be used. The thickness of the intermediate layer is preferably about 0.05 to 2 mm.

  As shown in FIG. 2, the fixing unit 20 of this embodiment has a pressure contact member 41 on the inner peripheral side of the pressure belt 22. The size of the pressure contact member 41 in the sheet width direction (depth direction in the figure) is at least equal to or greater than the heating width of the heating roller 21. The pressure contact member 41 includes a rigid pad 42 and an elastic pad 43 that are uniform in the width direction.

  The elastic pad 43 has a prismatic shape that is long in the depth direction in FIG. The elastic pad 43 is formed of a material having elasticity and heat resistance, such as silicone rubber and fluorine rubber. The hardness of the elastic pad 43 is preferably about 15 to 30 in terms of Asker C hardness. This is softer than the pressure belt 22 and the elastic layer 32 of the heating roller 21. The thickness of the elastic pad 43 (the size in the left-right direction in the figure) is preferably about 0.1 to 10 mm.

  The rigid pad 42 is a substantially rod-shaped member that covers three sides of the elastic pad 43. That is, a groove 42a is formed in the longitudinal direction of the rigid pad 42, and an elastic pad 43 is fixed therein. The rigid pad 42 is formed of a hard material as compared with the pressure belt 22 and the elastic layer 32 of the heating roller 21. In this embodiment, for example, polyphenylene sulfide, polyimide, resin such as liquid crystal polymer, metal such as aluminum and iron, or ceramics is suitable.

  As shown in FIG. 2, the pressure contact member 41 is disposed so that the groove 42a of the rigid pad 42 faces the heating roller 21 and is parallel to the axis thereof. The left end of the rigid pad 42 on the upper side (downstream in the sheet conveying direction) and the left side of the elastic pad 43 in the drawing are arranged so as to contact the pressure belt 22. The pressure belt 22 is pressed toward the heating roller 21 at those portions. In the following, the portions that come into contact with the pressure belt 22 are referred to as the front end portion 42 b of the rigid pad 42 and the front surface 43 a of the elastic pad 43.

  The distal end portion 42b of the rigid pad 42 is formed in a rounded convex shape. The front surface 43a of the elastic pad 43 is set to the same position as the convex end of the tip end portion 42b or slightly to the right. Of the rigid pad 42, a portion 42c below the elastic pad 43 in the drawing is formed only to a position far to the right from the front surface 43a of the elastic pad 43. This portion is only for holding the elastic pad 43. Accordingly, the rigid pad 42 is in contact with the pressure belt 22 only at a position downstream of the elastic pad 43 in the sheet conveyance direction.

  As shown in FIG. 2, the pressure belt 22 and the elastic pad 43 are deformed by the pressure contact member 41, and the pressure belt 22 is interposed between the front surface 43 a of the elastic pad 43 and the elastic layer 32 of the heating roller 21. A nip is formed through Further, the tip end portion 42 b is pushed into the heating roller 21 at the contact portion between the tip end portion 42 b of the rigid pad 42 and the heating roller 21. At any location, the pressure belt 22 is deformed into a shape between the pressure contact member 41 and the heating roller 21.

  As will be described later, the pressure contact member 41 is moved in the sheet conveyance direction (vertical direction in FIG. 2) as necessary. The range of the movement is within a range where both the front end portion 42 b of the rigid pad 42 and the front surface 43 a of the elastic pad 43 are pressed by the elastic layer 32 of the heating roller 21. Note that the elastic pad 43 may be formed integrally with a metal plate such as SUS, aluminum, or iron in order to improve assembling property or productivity. Thus, when using the elastic body pad 43 integrated with the metal plate, the metal plate is disposed on the side in contact with the inner surface of the groove 42a.

  Next, a configuration for moving the pressure contact member 41 will be described. Therefore, in this embodiment, a moving member 45 is provided above the pressure belt 22 in FIG. The moving member 45 includes a connecting member 46, a cam 47, and a motor 48. The connecting member 46 is rotatably attached to the cam 47 and the rigid pad 42 at both upper and lower ends in the figure.

  The cam 47 is attached to the rotating shaft 48a of the motor 48 at a position different from the place where the connecting member 46 is attached (the connecting place 47a). In FIG. 2, the connecting portion 47a is disposed on the left side of the rotating shaft 48a. Therefore, when the rotating shaft 48a of the motor 48 rotates, the cam 47 rotates around the rotating shaft 48a, and the connecting portion 47a is moved substantially up and down in the drawing. Thereby, the connecting member 46 moves in the vertical direction in the figure. The motor 48 is fixed in the machine.

  Further, as shown in FIG. 2, two positioning projections 51 and 52 are formed at the end of the rigid pad 42 in the width direction of the sheet, and are inserted into a long hole 53 provided in the machine. The two positioning projections 51 and 52 are movable within a range within the long hole 53. The long hole 53 is a through hole or groove long in the sheet conveyance direction. By cooperating with these members and the moving member 45, the press contact member 41 moves only in the sheet conveying direction (vertical direction in the figure) while maintaining the illustrated posture. The angle with respect to the conveyance direction of the sheet is not changed.

  FIG. 2 shows the arrangement of each part at a substantially central position in the movement range of the pressure contact member 41. This position is hereinafter referred to as a reference position. At this reference position, with respect to the cross-sectional shape of the heating roller 21, the elastic pad 43 abuts on a radial position whose tangent is the sheet conveying direction, and the leading end 42b of the rigid pad 42 is further downstream in the sheet conveying direction. Abut. A configuration similar to the combination of the positioning protrusions 51 and 52 and the elongated hole 53 is provided on both outer sides in the width direction of the press contact member 41.

  Further, in this embodiment, the sheet conveyance direction is upward from the bottom in FIG. 2, and a sheet information detection unit 55 is provided on the upstream side of the fixing unit 20 in the sheet conveyance direction. The sheet information detection unit 55 can detect sheet information such as transparency and thickness by irradiating light such as a laser onto a sheet being conveyed. Furthermore, the present embodiment also includes a control unit 56 for receiving the detection result by the sheet information detection unit 55 and appropriately driving the moving member 45.

  Next, the difference in the pressing state to the heating roller 21 according to the position of the pressure contact member 41 will be described. Each arrangement of the pressure contact members 41 shown in FIG. 3 is an arrangement in which (a) is the most downstream side and (c) is the most upstream side in the sheet conveyance direction. Is in position. The control unit 56 according to the present embodiment arranges the pressure contact member 41 at the position (a) in the figure when the conveyed sheet is a thick sheet. It is positioned at (b) for plain paper and at (c) for thin paper. In this figure, the pressure belt 22 is omitted. The reference position shown in FIG. 2 corresponds to the arrangement shown in FIG.

  In each of these arrangements, the amount of biting that the tip 42b of the rigid pad 42 is pushed into the heating roller 21 and the size of the nip formed between the front surface 43a of the elastic pad 43 and the elastic layer 32 of the heating roller 21 But they are different. This amount of biting corresponds to the nip outlet pressure. 3, (a) shows the biting amount Da and nip width Na, (b) shows the biting amount Db and nip width Nb, and (c) shows the biting amount Dc and nip width Nc.

Furthermore, the relationship between the biting amounts Da, Db, and Dc and the relationship between the nip widths Na, Nb, and Nc are as follows.
Da <Db <Dc
Na>Nb> Nc
It has become. However, even the smallest biting amount Da takes a positive value. Further, even the largest amount of bite Dc is about half the thickness of the elastic layer 32 of the heating roller 21. In other words, the downstream arrangement (a) has a smaller outlet pressure and a larger nip width than (b). In the arrangement of (c) on the upstream side, the outlet pressure is larger and the nip width is smaller than in (b).

  The fixability in the fixing unit 20 is generally better as the nip width is larger. Further, the separation between the fixing nip and the sheet becomes better as the outlet pressure of the nip increases. This is due to the elastic return action of the elastic layer 32 of the heating roller 21. That is, in each position shown in FIG. 3, the fixing property and the separating property are both moderate (b), whereas in (a), the fixing property is good and the separating property is slightly inferior. In (c), the fixing property is slightly inferior, but the separating property is good.

  On the other hand, when attention is paid to a sheet which is a member to be fixed, the fixing property is inferior as the thickness is increased. This is because a large amount of heat is required to sufficiently heat the sheet. Therefore, in the case of fixing to thick paper, it is preferable that the fixing unit 20 has excellent fixing properties. On the other hand, the cardboard is easily separated from the fixing unit 20. Therefore, separability is not so required. Therefore, when the sheet is thick paper, the position of the pressure contact member 41 is preferably set to the position (a) where the separation property is slightly inferior to the fixing property.

  The reverse is true if the sheet is thin. Excellent fixability, but easy to stick and difficult to separate. Therefore, when the sheet is thin paper, the fixing property may be slightly inferior, and it is preferable to set the position (c) where the separation property is excellent. In this way, the separability at the nip outlet is improved. Further, in the case of plain paper having a medium thickness, it is preferable that the arrangement (b) has a medium fixing property and separation property.

  That is, when it is determined from the detection result of the sheet information detection unit 55 that the sheet is thick paper, the control unit 56 moves the cam 47 and the motor 48 clockwise from the reference position shown in FIG. Rotate by a predetermined angle. Accordingly, the connecting member 46 is pulled upward in the drawing, and the pressure contact member 41 is moved upward in the drawing, that is, downstream in the sheet conveying direction. Thus, compared to the case of plain paper (FIG. 3B), when thick paper is fixed, the biting amount is smaller and the nip width is larger (FIG. 3A). To do.

  On the other hand, when it is determined from the detection result of the sheet information detection unit 55 that the paper is thin, the cam 47 and the motor 48 are determined in advance counterclockwise in the drawing from the reference position shown in FIG. Rotate only the angle. Thereby, the press contact member 41 is moved upstream in the sheet conveying direction. Then, as compared with the case of plain paper (FIG. 3B), the arrangement is such that the amount of biting is larger and the nip width is smaller (FIG. 3C). By doing so, it is possible to perform a good fixing process regardless of the thickness of the paper in both the fixing property and the separation property.

  The inventors conducted an experiment to confirm the separability by changing the fixing temperature, the fixing pressure, and the position of the pressure contact member 41. As a result, for example, when fixing processing to plain paper is performed at a fixing temperature of 190 ° C. and a fixing pressure of 180 N, the pressure contact member 41 is at the reference position or the position moved by 0.5 mm upstream. Both showed good separability. However, when the position moved 0.5 mm downstream from the reference position, the separability was not very good. That is, it was confirmed that the separability was improved when the amount of biting of the rigid pad 42 was increased. This tendency was the same even when the fixing temperature and the fixing pressure were changed.

  In addition to this, as a performance required for the fixing unit 20, there is a case where paper wrinkles are not generated on a passing sheet. Generally, paper wrinkles are more likely to occur as the sheet thickness is thinner. With thick paper, paper wrinkles are unlikely to occur. In addition, it has been found that paper wrinkles are less likely to occur by increasing the conveyance speed at both ends of the sheet in the width direction than at the center and increasing the difference within an appropriate range.

  In the experiments by the inventors, it has been found that the magnitude of the difference in the conveyance speed between the both ends and the center can be adjusted by the amount of biting of the rigid pad 42 into the heating roller 21. As shown in FIG. 4, the ratio of the difference in the conveyance speed between the both end portions and the central portion of the heating roller 21 to the central portion speed increases as the amount of biting of the rigid pad 42 increases. And as this ratio is in the range of about 0.3% to 0.7%, the paper wrinkles are less likely to occur as the ratio is larger. In the present embodiment, since the bite amount Dc is larger in the case of thin paper than in the case of plain paper, it is possible to provide the fixing unit 20 in which good paper wrinkle is hardly generated even in the case of thin paper.

  As described above in detail, according to the image forming apparatus 1 of the present embodiment, the pressure contact member 41 having the rigid pad 42 and the elastic pad 43 is pressed against the heating roller 21. Further, the arrangement of the pressure contact member 41 is adjusted by the moving member 45. Thereby, both the amount of biting of the rigid pad 42 and the nip width by the elastic pad 43 are changed, and the direction of the change is opposite. Therefore, the amount of biting and the nip width can be adjusted to be appropriate values according to the detection result by the sheet information detection unit 55. As a result, the image forming apparatus has a simple configuration and can realize good fixing performance and separation performance regardless of the type of paper.

In addition, this form is only a mere illustration and does not limit this invention at all. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof.
For example, the moving member is not limited to the motor and cam as shown in the above embodiment, and may be linearly driven by a cylinder or the like. In the above description, the adjustment is made in three stages of thick paper, plain paper, and thin paper, but it is also possible to adjust in a stepless manner according to the thickness of the paper. Further, the distance between the arrangements of the pressure contact members in the case of thick paper and plain paper and the distance in the case of thin paper and plain paper are not necessarily the same. Further, the photoconductor 11 is not limited to one, but can be applied to a color printer or the like.

1 is a schematic configuration diagram illustrating an image forming apparatus according to an embodiment. 1 is a schematic configuration diagram illustrating a fixing device of the present embodiment. It is explanatory drawing which shows the difference of the position of a press-contact member, and the press state to a heating roller by it. It is a graph which shows the relationship between the biting amount of a rigid body pad, and the difference of the conveyance speed in a center and both ends about the width direction.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 20 Fixing part 21 Heating roller 22 Pressure belt 32 Elastic layer 41 Pressure contact member 42 Rigid pad 43 Elastic body pad 45 Moving member 55 Sheet information detection part 56 Control part

Claims (4)

A fixing unit that has a pair of fixing members that rotate together and at least one of which is belt-shaped, and that fixes a toner image on the sheet by passing a sheet carrying a toner image between the pair of fixing members. In the device,
A pressure contact member disposed in contact with the back surface of the belt-shaped fixing member and press-contacting the belt-shaped fixing member toward the other fixing member;
A sheet thickness detecting member for detecting the thickness of the conveyed sheet at a position upstream of the pair of fixing members;
A moving member for moving the pressure contact member in a sheet conveying direction;
A control unit that causes the moving member to adjust the position of the pressure contact member based on the sheet thickness detected by the sheet thickness detection member;
The pressure contact member is:
A first pressing member that forms a nip portion of a finite length in the sheet conveying direction between the fixing members by pressing the belt-shaped fixing member against the other fixing member;
A second pressure contact member that is located downstream of the first pressure contact member in the sheet conveying direction and that projects from the first pressure contact member toward the other fixing member;
The controller is
When the sheet thickness is small, the length of the nip portion is shortened and the second pressure contact member to the other fixing member is moved by moving the pressure contact member to a downstream position in the sheet conveyance direction. Increase the amount of bite,
When the sheet is thick, the nip portion is lengthened by moving the pressure contact member to an upstream position in the sheet conveyance direction, and the second pressure contact member is moved to the other fixing member. A fixing device that reduces the amount of biting. The fixing device according to claim 1,
The other fixing member has an elastic layer;
The fixing device according to claim 1, wherein the elastic layer is harder than the first pressure contact member and softer than the second pressure contact member. The fixing device according to claim 1 or 2,
The fixing device according to claim 1, wherein the moving member moves the pressure contact member without changing an angle with respect to a sheet conveyance direction. The image forming unit includes a pair of fixing members that rotate together and at least one of which is belt-shaped, and passes between the pair of fixing members through a sheet carrying a toner image applied by the image forming unit. In an image forming apparatus having a fixing device that fixes a toner image on a sheet,
The fixing device includes:
A pressure contact member disposed in contact with the back surface of the belt-shaped fixing member and press-contacting the belt-shaped fixing member toward the other fixing member;
A sheet thickness detecting member for detecting the thickness of the conveyed sheet at a position upstream of the pair of fixing members;
A moving member for moving the pressure contact member in a sheet conveying direction;
A control unit that causes the moving member to adjust the position of the pressure contact member based on the sheet thickness detected by the sheet thickness detection member;
The pressure contact member is:
A first pressing member that forms a nip portion of a finite length in the sheet conveying direction between the fixing members by pressing the belt-shaped fixing member against the other fixing member;
A second pressure contact member that is located downstream of the first pressure contact member in the sheet conveying direction and that projects from the first pressure contact member toward the other fixing member;
The controller is
When the sheet thickness is small, the length of the nip portion is shortened and the second pressure contact member to the other fixing member is moved by moving the pressure contact member to a downstream position in the sheet conveyance direction. Increase the amount of bite,
When the sheet thickness is thick, the pressure contact member is moved to the upstream position in the sheet conveyance direction, thereby increasing the length of the nip portion and the second pressure contact member to the other fixing member. An image forming apparatus characterized in that an amount of biting is reduced.

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