JP2006113609A - Image fixing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image fixing device of the type in which an unfixed toner image is fixed to a recording sheet by pressing a pressure member and an auxiliary pressure pad against the elastic layer of a pressure roll, the image fixing device being designed such that a fixed image having satisfactory fixability and no image quality defect can be formed by optimizing the pressure contact force of the pressure member and auxiliary member against the heat roll. <P>SOLUTION: The pressure member 4 and the auxiliary pressure pad 3 are supported on a support frame 5 formed in the lengthwise direction of the heat roll 1. A pressure lever 7 connected such that the support frame 5 is freely rotatable are disposed at both lengthwise ends of the support frame 5. By pressing the one end of the pressure lever 7, the pressure member 4 and auxiliary pressure member 3 on the support frame 5 are pressed against the heat roll 1. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image fixing apparatus that heats and pressurizes an unfixed toner image transferred onto a recording sheet and fixes the toner image on the recording sheet, and more specifically, a heating roll including a heating unit and the heating roll. The present invention relates to an improvement in an image fixing apparatus that performs fixing by inserting a recording sheet between an endless fixing belt that is pressed against the fixing belt.

Japanese Patent Application Laid-Open No. 11-133776

  Conventionally, as this type of image fixing device, the one disclosed in Patent Document 1 is known. As shown in FIG. 15, the apparatus includes a heating roll 101 in which a halogen lamp 100 as a heater is built in, and an endless fixing belt 102 that is rotated by the heating roll 101. By inserting the recording sheet P carrying the image T between the heating roll 101 and the fixing belt 102, the toner softened by the surface temperature of the heating roll 101 penetrates between the fibers of the recording sheet P, and the undetermined state. The received toner image T is firmly fixed to the recording sheet P.

  In particular, in this image fixing apparatus, a wide auxiliary pressure pad 103 made of a soft elastic member is provided inside the fixing belt 102, and a pressure member 104 made of a metal plate or the like is adjacent to the auxiliary pressure pad 103. The auxiliary pressure pad 103 and the pressure member 104 are pressed against the heating roll 101 so that the fixing belt 102 is wound around a part of the peripheral surface of the heating roll 101. ing. Accordingly, a region for sandwiching the recording sheet P between the heating roll 101 and the fixing belt 102 (hereinafter, this region is referred to as a “nip region”) can be formed relatively long. Thus, heat and pressure can be applied for a long time, even when the amount of unfixed toner T is large as in a color image or when the conveyance speed of the recording sheet P passing through the fixing device is high. It is possible to satisfactorily fix the unfixed toner image T to the recording sheet P.

  Further, in this image fixing apparatus, in order to prevent the recording sheet P that has passed through the nip region from sticking to the heating roll 101 due to the adhesive force of the molten toner, the peripheral surface of the heating roll 101 has a predetermined thickness or more. The elastic member 105 is covered, and the pressure member 104 provided at the end portion of the nip region is pressed against the heating roll 101 with a predetermined pressure contact force to cause the elastic layer 105 to be distorted. Even without using the apparatus, the action of peeling the recording sheet P from the heating roll 101 (hereinafter, the peeling action of the recording sheet is called “self-stripping action”) is promoted.

  Further, in this image fixing apparatus, the heating roll 101 is thinned in order to shorten the warm-up time until the surface temperature of the heating roll 101 reaches from the room temperature to the fixable temperature. The thinning of 101 is advantageous also when a color image is fast-fixed using a small-diameter heating roll whose heat capacity must be small.

  However, in the conventional image fixing apparatus configured as described above, when the pressing member is biased toward the heating roll with a large force so as to exhibit the self-stripping action, the thin heating roll is axially moved. Since it bends, there is a problem in that a uniform pressure cannot be applied to the recording sheet along the axial direction of the heating roll, and a fixing failure of the toner image tends to occur. That is, since both ends of the heating roll are supported, a sufficient pressing force acts between the fixing belt and the heating roll at both ends in the axial direction, but the heating roll bends at the center in the axial direction. The pressure contact force was reduced by that amount, and the toner image was liable to be poorly fixed at the center of the recording sheet.

  On the other hand, the self-stripping action is more prominent as the amount of strain of the elastic layer formed on the outer peripheral surface of the heating roll increases. Therefore, in order to promote the action, the pressure member is pressed with a larger force. It is preferable to press the However, if the distortion of the elastic layer of the heating roll by the pressure member becomes large, the recording sheet that has passed through the fixing device will be strongly curled accordingly, so that the recording finally discharged from the image forming apparatus such as a printer In consideration of the finished state of the sheet, it is necessary to provide a decurler for reducing the curling of the recording sheet after the fixing device, resulting in a complicated configuration of the entire image forming apparatus and an increase in manufacturing cost. There was a point. On the other hand, when the pressing force by the pressure member is reduced, the curling of the recording sheet is reduced, but the self-stripping action is not fully exhibited, and the peeling for peeling off the recording sheet attached to the heating roll. There is a problem that an extra structure such as a nail is required, and the fixing device itself is increased in size.

  Further, in such an image fixing device, when the pressure distribution in the nip region is taken into consideration, the pressure distribution curve formed by the auxiliary pressure pad and the pressure distribution curve formed by the pressure member are smooth. It is necessary to draw a continuous curve. If this pressure distribution curve is discontinuous at the boundary between the auxiliary pressure pad and the pressure member, image defects such as rubbing in the process of fixing the toner image on the recording sheet will occur. For this reason, the balance of pressure exerted on the heating roll by the pressure member and the auxiliary pressure pad is extremely important in the image fixing apparatus having the above-described configuration. If this balance is lost, a fixing defect or an image quality defect occurs. There was a problem.

  The present invention has been made in view of such problems, and an object thereof is to fix a non-fixed toner image on a recording sheet by pressing a pressure member and an auxiliary pressure pad against an elastic layer of a heating roll. Type image fixing device, by optimizing the pressure contact force of the pressure member and the auxiliary pressure member to the heating roll, an image fixing device having a good fixing property and having no image quality defect Is to provide.

  That is, the present invention has a heating roll having an elastic layer on the outer peripheral surface and a built-in heat generating means that is driven to rotate, an endless fixing belt rotated by the heating roll, and an unfixed toner image. In order to form a nip region of the recording sheet conveyed in the above state, an auxiliary pressure pad that presses the fixing belt toward the heating roll, and a downstream side of the auxiliary pressure pad in the recording sheet conveyance direction. In order to achieve the above object, it is assumed that the image fixing device is disposed adjacently and is configured to include a pressure member that deforms the elastic layer of the heating roll by being pressed against the heating roll via the fixing belt. The following configuration is adopted for the image fixing apparatus.

  The image fixing apparatus according to the present invention supports the pressure member and the auxiliary pressure pad on a support frame formed along the longitudinal direction of the heating roll, and the pressure lever in which the support frame is rotatably connected. Are arranged at both ends in the longitudinal direction of the support frame, and by pressing one end of the pressure lever, the pressure member and the auxiliary pressure member on the support frame are pressed against the heating roll. It is a feature.

  According to such technical means, the support frame supporting the pressure member and the auxiliary pressure pad is rotatably connected to the pressure lever, and the one end of the pressure lever is biased to Since the pressure member and the auxiliary pressure pad are configured to be pressed against the heating roll, even if there is variation in the fulcrum position of the pressure lever and the mounting position of the support frame with respect to the pressure lever, Although the magnitude of the pressure contact force exerted by the pressure member and the auxiliary pressure pad on the heating roll changes due to the relative rotation of the support frame and the pressure lever, the balance of the pressure contact force between them is The pressure member and the auxiliary pressure pad can be pressed against the heating roll while maintaining a constant balance without changing. Accordingly, it is possible to always optimize the pressure distribution between the heating roll and the fixing belt in the recording sheet conveyance direction, and to prevent the occurrence of fixing failure or image quality failure as much as possible.

  As described above, according to the image fixing device of the present invention, in an image fixing device of a type in which a pressure member and an auxiliary pressure pad are pressed against an elastic layer of a heating roll to fix an unfixed toner image on a recording sheet. By optimizing the pressure contact force of the pressure member and the auxiliary pressure member to the heating roll, it is possible to obtain a fixed image with good fixability and no image quality defect.

  Hereinafter, an image fixing device of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 shows an embodiment of an image fixing apparatus to which the present invention is applicable. As shown in the figure, this image fixing apparatus includes a heating roll 1 incorporating halogen lamps 11a and 11b as heaters, an endless fixing belt 2 rotated by the heating roll 1, and a recording sheet P. An auxiliary pressure pad 3 that presses the fixing belt 2 toward the heating roll 1 so as to form a nip region of the recording sheet P, and is disposed adjacent to the auxiliary pressure pad 3 on the downstream side in the conveyance direction of the recording sheet P. A pressure member 4 that presses against the heating roll 1 via the fixing belt 2, and a recording sheet P carrying an unfixed toner image T is inserted between the heating roll 1 and the fixing belt 2. It has become.

  First, the heating roll 1 covers a metal core 12 with an elastic body layer 13 and houses a pair of halogen lamps 11 a and 11 b having different outputs inside the core 12. An aluminum cylinder is used. The elastic body layer 13 includes a base layer having heat resistance and a top coat layer for preventing adhesion of melted toner. The top coat layer is dip-coated on the base layer so that the periphery of the heating roll 1 is formed. The surface is kept in a surface state close to a mirror surface. In addition, a drive motor (not shown) is connected to the heating roll 1 and is driven to rotate at a predetermined angular velocity.

  On the other hand, the fixing belt 2 is a polyimide film, and is provided so as to surround the support frame 5 to which the pressure member 4 and the auxiliary pressure pad 3 are attached. It is configured to rotate freely. Guide members 20 and 21 having curved sliding surfaces are fixed to both sides of the support frame 5, respectively. The fixing belt 2 is supported from the inside by the guide members 20 and 21, and rotates along a predetermined path. It is supposed to be.

  The support frame 5 includes a frame body 5a formed by bending a metal plate into a channel shape, and a support plate 5b fixed so as to close a groove of the frame body 5a. It is formed slightly longer than the width. The pressure member 4 and the auxiliary pressure pad 3 are provided on the support plate 5 b and are in pressure contact with the heating roll 1 from the inside of the fixing belt 2. Then, by pressing the pressure member 4 and the auxiliary pressure pad 3 against the heating roll 1, the fixing belt 2 is forcibly wound around the circumferential surface of the heating roll 1, and the nip region of the recording sheet P is predetermined. Only the length of is formed. In order to reduce the frictional resistance between the auxiliary pressure pad 3 and the pressure member 4 and the fixing belt 2, a resin sheet having a low coefficient of friction such as a Teflon (registered trademark) sheet is interposed therebetween. .

  The auxiliary pressure pad 3 is disposed adjacent to the upstream side in the conveyance direction of the recording sheet P with respect to the pressure member 4, that is, adjacent to the start end side of the nip region, so as to improve the pressure contact force of the fixing belt 2 to the heating roll 1. The fixing belt 2 is in pressure contact with the heating roll 1 from the back side. The auxiliary pressure pad 3 is provided with an elastic layer 31 made of silicon rubber on a channel-shaped holding member 30, and the holding member 30 is biased toward the heating roll 1 by a coil spring 32. Has been. A plurality of such coil springs 32 are arranged along the width direction of the fixing belt, and the heating roll 1 is bent along the axial direction by the pressure contact between the pressure member 4 and the auxiliary pressure pad 3. In addition, the biasing force of each coil spring 32 is adjusted so that the auxiliary pressure pad 3 can exert a uniform pressure contact force along the axial direction.

  A spring holder 33 is provided below the auxiliary pressure pad 3, and the spring holder 33 holds the coil spring 32 and positions the auxiliary pressure pad 3 with respect to the support frame 5. Yes. A plurality of cylindrical boss portions 34 project from the spring holder 33 along the width direction of the fixing belt 2, and a coil spring 32 is erected in the boss portions 34. The support plate 5b has a plurality of positioning holes corresponding to the boss portions 34 of the spring holder 33. By fitting the boss portions 34 into the positioning holes, the spring holder 33 is supported by the support frame. 5 for positioning. Further, a guide wall 35 is erected on the side edge portion of the spring holder 33 along the longitudinal direction, and the auxiliary pressure pad 3 is positioned in the rotation direction of the fixing belt 2 to position the guide wall 35 and the adjacent pressure. It is made by the member 4.

  On the other hand, the pressure member 4 is a plate-like metal member having the same cross-sectional shape along the width direction of the fixing belt 2, and protrudes to the auxiliary pressure pad 3 side at the tip that presses against the inside of the fixing belt 2. A pressing portion 40 is formed. Further, leg portions 41 as shown in FIG. 2 are formed at a predetermined interval at the end opposite to the pressing portion 40, and these leg portions 41 are formed in the mounting holes 50 formed in the support plate 5b. The pressurizing member 4 is positioned on the support frame 5 by fitting.

  FIG. 3 shows an outline of a configuration for bringing the heating roll 1 and the fixing belt 2 into pressure contact with each other. A pair of fixed frames 6 are provided at both ends of the heating roll 1 in the axial direction, and the heating roll 1 is supported by the fixed frame 6 via a bearing (not shown). On the other hand, a pair of movable frames 7 are provided at both ends of the fixing belt 2 in the axial direction, and both ends of the support frame 5 positioned inside the fixing belt 2 are fixed to the movable frame 7 by a closing cap (not shown). ing. A stud 60 is erected on the fixed frame 6, while a hook portion 70 is formed on the movable frame 7 so as to be hooked on the stud 60, and the movable frame 7 can swing around the stud 60. Has been. Further, a flange portion 71 is formed in the vicinity of the end portion of the movable frame 7 opposite to the hook portion 70, and the flange portion 71 is pressurized so that the front end can be hooked on the locking portion 61 on the fixed frame 6 side. A rod 72 is provided. A pressure spring 73 is fitted around the pressure rod 72 and is configured to press the flange portion 71 in a direction in which the fixing belt 2 is pressed against the heating roll 1. Therefore, when the movable frame 7 is swung around the stud 60 of the fixed frame 6 and the tip of the pressure rod 72 is hooked on the engaging portion 61 of the fixed frame 6, the pressure spring 73 is moved to the flange portion of the movable frame 7. 71 is pressed, whereby the pressure member 4 and the auxiliary pressure pad 3 are pressed against the heating roll 1 via the fixing belt 2, and the fixing belt 2 and the heating roll 1 form a predetermined nip region.

  In the image fixing apparatus of the present embodiment configured as described above, when the recording sheet P is inserted through the nip region formed by the pressure contact between the heating roll 1 and the fixing belt 2, the recording sheet is used. The unfixed toner image T carried on P is heated and pressurized and fixed on the recording sheet P. In particular, in this fixing device, since the metal pressure member 4 is pressed against the heating roll 1 at the end of the nip region, the elastic body layer 13 of the heating roll 1 is pressed against the pressure member and is locally elastic. Deformation occurs, and the action of separating the recording sheet P passing through this portion from the heating roll 1, that is, the self-stripping action is exhibited. For this reason, the recording sheet P that has passed through the nip region can be naturally peeled from the heating roll 1 without using a forced peeling means such as a peeling claw.

  However, when the pressure distribution between the heating roll 1 and the pressure member 4 is taken into account, the heating roll 1 is supported by the fixed frame 6 at both shaft ends, and therefore the pressure distribution between the two. As shown in FIG. 4, it tends to be smaller near the center in the longitudinal direction than near the shaft end of the heating roll 1. Further, since the core 12 of the heating roll 1 is composed of a thin aluminum sleeve, it is easy to bend along the axial direction when the pressing member 4 is brought into pressure contact with the heating roll 1, and this is also shown in FIG. This is one reason that pressure distribution tends to occur. For this reason, when the pressing portion 40 at the tip of the pressure member 4 is formed in a straight line parallel to the rotation axis of the heating roll 1, the pressure is insufficient near the center in the longitudinal direction of the heating roll 1, and the toner image The fixing strength tends to be insufficient.

  Therefore, in order to make the pressure distribution uniform along the axial direction of the heating roll 1, as shown in FIG. 5, the pressing portion 40 of the pressing member 4 is formed in a convex curved shape along the longitudinal direction, What is necessary is just to comprise so that the pressurization member 4 may be strongly pressed on this heating roll 1 by the center vicinity rather than the axial end vicinity. However, in this case, the convex curved surface at the tip of the pressure member must be formed so that the pressure distribution between the heating roll and the pressure member is uniform, and the shape of the curved surface is specified and processed. Requires a great deal of labor and cost.

  Therefore, in the fixing device of this embodiment, as shown in FIG. 2, the pressing portion 40 of the pressing member 4 is formed in a straight line parallel to the rotation axis of the heating roll 1, while the pressing member 4 is erected. A shim 42 made of a thin metal plate is interposed between the support plate 5b and the lower end edge of the pressurizing member 4, and the lower end side of the pressurizing member 4 is pseudo-stepped using the shim 42. Thus, the pressing portion 40 pressed against the heating roll 1 is slightly deformed into a convex curved surface. That is, the number of shims 42 to be interposed is set near the center in the longitudinal direction of the pressure member 4, while the number of shims 42 is decreased as approaching both ends in the longitudinal direction, and the shims 42 are sandwiched. By pressing the pressing member 4 against the support plate 5b, the pressing member 4 is slightly elastically deformed, and the pressing portion 40 is deformed into a convex curved surface as shown in FIG. As described above, if the pressing member 40 is elastically deformed so that the pressing portion 40 is arranged in a convex curved surface shape, the number of shims 42 to be interposed can be changed at each position along the longitudinal direction. The shape of the convex curved surface of the pressing portion 40 can be adjusted arbitrarily and easily, and the work man-hours and time for obtaining a uniform pressure distribution can be greatly saved.

  If the number of shims 42 sandwiched between the pressure member 4 and the support plate 5b is determined, the lower end side of the pressure member 4 is stepped in accordance with the number of the shims 42 as shown in FIG. In the trial production, adjustment is made so that a uniform pressure distribution is obtained between the pressure member 4 and the heating roll 1 using the shim 42, and the number of the shims 42 determined at the time of mass production. Reflecting the above, the lower end side of the pressure member 4 may be formed in a staircase shape. Thereby, it is not necessary to perform troublesome curved surface processing on the pressing portion 40 of the pressing member 4, and a uniform pressure distribution between the pressing member 4 and the heating roll 1 can be realized inexpensively and easily. It becomes possible.

  On the other hand, as another method for making the pressure distribution between the pressure member 4 and the heating roll 1 uniform, a coil spring, between the pressure member 4 and the support plate 5b, as shown in FIG. It is also effective to dispose an elastic member 43 such as a leaf spring or a rubber piece and change the pressing force of the elastic member 43 along the longitudinal direction of the support plate 5b. Specifically, by changing the urging force of a plurality of elastic members 43 arranged along the longitudinal direction of the pressure member 4, for example, the spring force of the coil spring, the thickness of the rubber piece, etc., the pressure member A pressing force f1 is applied in the vicinity of the center of 4, and pressing forces f2 and f3 that are successively smaller than f1 as the two ends of the pressing member 4 are approached. This compensates the pressing force that tends to be insufficient near the center of the pressing member 4 in the longitudinal direction, and makes the pressure distribution along the longitudinal direction between the pressing member 4 and the heating roll 1 substantially uniform. Is possible.

  In the fixing device of this embodiment, as the pressing force of the pressure member 4 against the heating roll 1 increases, the elastic body layer 13 of the heating roll 1 is greatly distorted, and the leading edge of the recording sheet P that has passed through the pressure member 4 is deformed. A so-called self-stripping action that spontaneously peels from the heating roll 1 occurs remarkably. On the other hand, as the pressure contact force of the pressure member 4 increases, the warping of the recording sheet P that has passed through the fixing device tends to increase, and a device for eliminating the warping of the recording sheet P is required after the fixing device. End up. Such warpage of the recording sheet P after passing through the fixing device is likely to occur in the recording sheet P having a large basis weight such as cardboard, but the pressure contact force of the pressure member 4 is reduced in order to reduce the warpage. In addition, the self-stripping action is not sufficiently exerted on the recording sheet P having a small basis weight, such as thin paper, and the trouble that the recording sheet P winds around the heating roll 1 occurs.

  As means for effectively avoiding such trouble, it is preferable to apply a bias voltage from the power source 44 to the pressure member 4 as shown in FIG. The pressure member 4 is made of a conductive metal such as stainless steel, and the fixing belt 2 is made of insulating polyimide. Therefore, when a bias voltage is applied to the pressure member 4, the nip An electrostatic attraction force acts on the recording sheet P that has passed through the region, and the recording sheet P is attracted to the fixing belt 2 side. Accordingly, even if the pressure contact force of the pressure member 4 to the heating roll 1 is small and the above-described self-stripping action is not sufficiently exhibited, the recording sheet P that has passed through the nip region does not wrap around the heating roll 1. Since the pressure contact force of the pressure member 4 that is discharged from the fixing device can be set small, it is possible to reduce the warp that occurs in the recording sheet P that has passed through the fixing device.

  The bias voltage applied to the pressing member 4 is preferably changed according to the type of the recording sheet P. For example, the bias voltage is set to be small for a thick paper in which the peeling effect due to the self-stripping action is remarkably generated, and the bias voltage is set to be large for a thin paper where the peeling effect due to the self-stripping action is difficult to occur. As a result, the recording sheet P can be stably peeled from the heating roll 1 while compensating for the shortage of the self-stripping action with the electrostatic attraction force.

  On the other hand, in this fixing device, the pressure member 4 presses the heating roll 1 with a greater force than the auxiliary pressure pad 3, but in order to obtain a good fixed image without disturbance, FIG. As shown, the pressure contact force of the pressure member 4 to the heating roll 1 and the pressure contact force of the auxiliary pressure pad 3 must be smoothly continuous. If any one of the pressure contact forces becomes larger than the initially determined value, the balance of the pressure contact forces between the two members is lost, and as shown in FIG. Since a discontinuous boundary is generated between the contact pressure of the auxiliary pressure pad 3 and the auxiliary pressure pad 3, the nip area is divided into two parts, and the toner image is rubbed on the recording sheet P.

  A structure for press-contacting the heating roll 1 and the fixing belt 2 is shown in FIG. 3. In this structure, the pressure member 4 and the auxiliary force are applied by utilizing the “leverage principle” with the stud 60 of the fixed frame 6 as a fulcrum. The pressure pad 3 is pressed against the heating roll 1, and the support frame 5 holding the pressure member 4 and the auxiliary pressure pad 3 is fixed to the movable frame 7. For this reason, when an error occurs in the standing position of the stud 60 with respect to the fixed frame 6 or the formation position of the hook portion 70 in the movable frame 7, the distance from the stud 60 as a fulcrum to the pressure contact position of the pressing member 4, auxiliary pressurization. There is a concern that the distance to the press contact position of the pad 3 changes, and the balance of the press contact force of these members against the heating roll 1 is lost.

  Therefore, in order to avoid such a problem, as shown in FIG. 11, the support frame 5 holding the pressure member 4 and the auxiliary pressure pad 3 is rotatable with respect to the movable frame (pressure lever) 7. Is preferably maintained. Specifically, a support shaft 74 is provided upright at both ends in the longitudinal direction of the support frame 5, and the support shaft 74 is supported by the movable frame 7. With this configuration, since the support frame 5 can freely rotate around the support shaft 74, the distance from the tail stud 60 to the pressure contact position of the pressure member 4, and the pressure contact position of the auxiliary pressure pad 3. Even if the distance of the pressure has changed, the balance between the pressure contact force of the pressure member 4 and the pressure contact force of the auxiliary pressure pad 3 is not lost. As shown in FIG. The force distribution can be made smoothly continuous.

  Further, when a thick paper is used as the recording sheet P, in the structure shown in FIG. 3, the thick paper enters between the auxiliary pressure pad 3 and the heating roll 1, and the auxiliary pressure pad 3 corresponds to the thickness of the thick paper. When the pressure member 4 is pushed down by a distance, the pressure member 4 located farther from the auxiliary pressure pad 3 with respect to the stud 60 is pushed down by a larger distance than the auxiliary pressure pad 3. There is a concern of shortage. However, in the pressure contact structure shown in FIG. 11, the support frame 5 is arranged so as to be rotatable about the support shaft 74, so that even if the auxiliary pressure pad 3 is pushed down by the recording sheet P, the support frame 5 rotates. By doing so, the influence on the pressure contact force of the pressure member 4 can be eliminated, and even when the toner image is fixed to the thick paper, the pressure member 4 is appropriately pressed against the heating roll 1. It becomes possible to make it.

  Furthermore, in the case of the pressure contact structure shown in FIG. 11, the balance between the pressure contact force of the pressure member 4 and the pressure contact force of the auxiliary pressure pad 3 is changed by adjusting the standing position of the support shaft 74 with respect to the support frame 5. It is also possible. That is, as shown in FIG. 12, since the urging force of the pressure spring 73 is transmitted from the movable frame 7 to the support frame 5 via the support shaft 74, the distance from the support shaft 74 to the pressure member 4 is expressed as d1. When the distance to the auxiliary pressure pad 3 is d2, by changing the ratio of d1 and d2, the pressure contact force F1 of the pressure member 4 to the heating roll 1 and the pressure contact force F2 of the auxiliary pressure pad 3 are changed. The balance can be changed. For example, if F1 is set to be twice F2, d1 may be ½ of d2. This makes it possible to easily control the balance of the pressure contact force between the pressure member 4 and the auxiliary pressure pad 3.

  By the way, when a full color toner image is fixed to an OHP sheet or a thick paper, it is necessary to apply sufficient heat energy to the recording sheet P in order to improve the color developability of the fixed toner image. is there. For this reason, conventionally, when fixing a toner image to an OHP sheet or thick paper, the rotation speed of the heating roll is switched to a lower speed than the normal rotation speed for plain paper or the like. Measures are taken to fix the toner image while applying sufficient thermal energy to the sheet or cardboard. For example, when a recorded image is formed using an OHP sheet as a recording sheet in a copying machine, a printer, or the like, as shown in FIG. 13, a heating roll is set immediately before the first OHP sheet enters the fixing device. The heating roll starts rotating at the same speed until the rotation starts at a low speed and the final OHP sheet passes through the fixing device.

  In a fixing device of a type that fixes a toner image by pressing a solid pressure roll against the heating roll, the pressure roll has a certain heat capacity. While the energy is accumulated in the pressure roll and the OHP sheet is continuously sent, the surface temperature of the pressure roll will be extremely lowered even if the heating roll is always rotated at a low speed. And sufficient heat energy can be applied to the OHP sheet. However, in the fixing device of this embodiment, the fixing belt 2 is an extremely thin resin film, and the members inside the fixing belt 2 have a small volume and none of them has a large heat capacity. When the OHP sheet continuously passes while rotating at a low speed, the temperatures of the fixing belt 2, the pressure member 4, and the auxiliary pressure pad 3 are rapidly decreased. For this reason, there is a concern that the heat energy applied to the OHP sheet from the heating roll 1 is absorbed by the fixing belt 2 and the toner image on the OHP sheet cannot be sufficiently melted.

  In order to avoid such a problem, it is necessary to give sufficient heat energy to the fixing belt 2, the pressure member 4, and the auxiliary pressure pad 3 in a state where the recording sheet P is not inserted into the fixing device. For this purpose, it is effective to increase the rotation speed of the heating roll 1 in the time zone between the preceding and following OHP sheets even when the OHP sheets are continuously sent to the fixing device. Specifically, as shown in FIG. 14, while the OHP sheet passes through the fixing device, the rotation speed of the heating roll 1 is kept low, while the trailing edge of the first OHP sheet leaves the fixing device. If it is determined that the heating roller 1 has been rotated, the rotation speed of the heating roll 1 is immediately switched to a normal speed for plain paper or the like, and if it is determined that the leading edge of the second OHP sheet has approached the fixing device, Immediately switch the rotation speed of the heating roll 1 to the low speed for the OHP sheet. As described above, even when the OHP sheets are continuously sent, if the rotational speed of the heating roll 1 is switched to a high speed between the OHP sheets sent before and after, the number of the OHP sheets increases more in this time zone. It is possible to prevent the surface temperature of the pressure member 4 and the auxiliary pressure pad 3 from drastically lowering when the next OHP sheet passes through. It becomes. Therefore, even when a full color image is continuously formed on a recording sheet P having a large heat capacity such as an OHP sheet or a thick paper sheet, it is possible to obtain a fixed image with good color development and fixing strength. .

1 is a front sectional view showing an embodiment of an image fixing device to which the present invention is applied. It is a side exploded view which shows the attachment state of the pressurization member which concerns on an Example. It is a front view which shows the structure for pressing the heating roll and fixing belt which concern on an Example. It is explanatory drawing which shows the press-contact state of a pressurization member and a heating roll, and the pressure distribution at that time. It is explanatory drawing which shows pressure distribution at the time of forming the press-contact surface to the heating roll of a pressurization member in a convex curve shape. It is a side view which shows the example which formed the lower end side of the pressurization member in step shape. It is a side view which shows the example which has arrange | positioned the elastic member between the lower end side of a pressurization member, and a support plate. FIG. 6 is a front cross-sectional view illustrating an example in which a bias voltage is applied to a pressure member for promoting peeling of a recording sheet from a heating roll. It is a graph which shows the favorable state of the pressure distribution of a pressurization member and an auxiliary pressurization pad. It is a graph which shows the state with a bad pressure distribution of a pressurization member and an auxiliary pressurization pad. It is a front view which shows the example which supported the support frame rotatably with respect to the movable frame. It is a figure explaining the relationship between arrangement | positioning of a spindle, and the balance of the press-contact force of a pressurization member and an auxiliary | assistant press pad when a support frame is rotatably supported with respect to a movable frame. It is a timing chart which shows the change of the rotational speed of a heating roll at the time of inserting an OHP sheet in the conventional fixing device. 6 is a timing chart showing changes in the rotation speed of the heating roll when an OHP sheet is inserted through the fixing device according to the example. It is a front sectional view showing an example of a conventional image fixing device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Heating roll, 2 ... Fixing belt, 3 ... Auxiliary pressure pad, 4 ... Pressure member, 5 ... Support frame, 5b ... Support plate

Claims (1)

A heating roll having an elastic layer on the outer peripheral surface and having a heating means built therein and driven to rotate, an endless fixing belt rotated by the heating roll, and an unfixed toner image carried An auxiliary pressure pad that presses the fixing belt toward the heating roll in order to form a nip area of the coming recording sheet, and is arranged adjacent to the auxiliary pressure pad on the downstream side in the recording sheet conveyance direction, and the fixing In an image fixing device comprising a pressure member that is pressed against a heating roll via a belt and deforms an elastic layer of the heating roll,
The pressure member and the auxiliary pressure pad are supported on a support frame formed along the longitudinal direction of the heating roll, and a pressure lever to which the support frame is rotatably connected is disposed in the longitudinal direction of the support frame. An image fixing apparatus, characterized in that the pressure fixing member and the auxiliary pressure member on the support frame are pressed against a heating roll by being arranged at both ends and urging one end of the pressure lever.

Images