JP2018010221A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus which can suppress variations in the longitudinal direction in the width of a fixing nip part and suppress toner adhesion to a recording material rear end in a transfer nip part.SOLUTION: An image forming apparatus includes: a transfer member which forms a transfer nip part for transferring a toner image to a recording material; a heating member which heats the recording material to which the toner image is transferred; a facing member which faces the heating member and forms a fixing nip part holding and conveying the recording material together with the heating member; and a contact member which is provided in a recording material conveyance path between the transfer nip part and the fixing nip part and contacts a surface opposite to a surface where the toner image is provided in the recording material. When viewed from the conveyance direction of the recording material, the fixing nip part has such a shape that the center part protrudes to the side of the facing member with respect to the end in the longitudinal direction orthogonal to the conveyance direction of the recording material, and the contact member has such a shape that the center part in the longitudinal direction protrudes to the side opposite to the direction of protrusion of the fixing nip part.SELECTED DRAWING: Figure 5

Description

  The present invention relates to an image forming apparatus such as a copying machine, a laser beam printer, and an LED (light emitting diode) printer.

  There is known a film heating type fixing device that uses a film having a small heat capacity as a fixing member and shortens the time required to rise to a predetermined temperature. However, in the case of a film heating type fixing device, the fixing quality may deteriorate at the center in the longitudinal direction perpendicular to the recording material conveyance direction.

  That is, in the film heating type fixing device, the backup member that presses the fixing film against the pressure roller and the pressure roller are urged by a pressure mechanism provided at both ends in the longitudinal direction. At this time, the pressure in the vicinity of the central portion in the longitudinal direction is bent and the pressure is lowered, and the width of the fixing nip portion is smaller in the central portion in the longitudinal direction than both ends. If there is a large variation in the width of the fixing nip portion in the longitudinal direction, the fixing property of the recording material passing through the fixing nip portion varies in the longitudinal direction, and the fixing quality is lowered.

  In order to make the width of the fixing nip portion uniform in the longitudinal direction, the pressure in the central portion in the longitudinal direction is lowered by making the fixing nip convex toward the pressure roller when viewed from the recording material conveyance direction. Is known (Patent Documents 1 and 2).

  On the other hand, a configuration is known in which the recording material conveyance path from the transfer nip portion to the fixing nip portion is C-shaped (C path) in order to reduce the size of the apparatus (Patent Document 3).

JP2013-73198A Japanese Patent Application No. 2012-270303 JP 2013-164537 A

  Here, when the fixing nip portion is convex when viewed from the recording material conveyance direction (Patent Documents 1 and 2), there is a problem that toner adheres to the rear end of the recording material released from the transfer nip portion. This will be described with reference to FIGS.

  As shown in FIG. 12, when the fixing nip portion Nf is flat in the longitudinal direction when the fixing nip portion Nf is viewed from the transfer nip portion Nt in the recording material conveyance direction, the trailing edge of the recording material passes through the transfer nip portion Nt. The longitudinal shape of the rear end of the recording material is also flat. For this reason, the contact pressure between the rear end of the recording material and the surface of the transfer roller 5 is dispersed throughout in the longitudinal direction. The amount of toner adhering at each position in the longitudinal direction of the rear end of the recording material is small, hardly noticeable, and causes no problem.

  However, as shown in FIG. 13, the heater holder 241 formed in a curved section convex in the same direction as the bending direction of the pressure roller 23 is urged by the spring B to the pressure roller 23 via the fixing film 24. When the fixing device 6 is used, the following problems occur. In other words, the longitudinal shape of the trailing edge of the recording material when the trailing edge of the recording material passes through the transfer nip portion Nt is a wavy shape as shown in FIG.

  This is because the fixing nip portion has a central portion in the longitudinal direction that is convex toward the pressure roller when viewed from the recording material conveyance direction, and the recording material also has a central portion in the longitudinal direction that is convex toward the pressure roller. This is because the rear end of the recording material is deformed by the transfer roller having a uniform shape in the longitudinal direction.

  As described above, when the trailing edge of the recording material is in contact with the transfer roller 5 in a state where the recording material is undulated in the longitudinal direction, the contact pressure locally increases, so that the amount of toner scraped off is larger than that in other portions. In other words, the rear end edge becomes dirty (in FIG. 13A, the rear end edge is indicated by hatching).

  Such trailing edge contamination becomes conspicuous when the recording material conveyance path from the transfer portion to the fixing portion is C-shaped (C pass) for downsizing the apparatus (FIG. 14). This is because the restoring force of the recording material increases because the degree of bending of the conveyance path is strong.

  An object of the present invention is to provide an image forming apparatus capable of suppressing variations in the longitudinal direction of the width of the fixing nip portion and suppressing toner adhesion to the rear end of the recording material in the transfer nip portion.

  In order to achieve the above object, an image forming apparatus according to the present invention includes a transfer member that forms a transfer nip portion that transfers a toner image onto a recording material, and a heating member that heats the recording material onto which the toner image has been transferred. An opposing member that forms a fixing nip portion that faces the heating member and sandwiches and conveys the recording material together with the heating member, and a recording material conveyance path between the transfer nip portion and the fixing nip portion, A contact member that contacts a surface of the recording material opposite to the surface on which the toner image is provided, and the fixing nip portion is orthogonal to the recording material conveyance direction when viewed from the recording material conveyance direction. The center portion of the longitudinal direction protrudes toward the opposing member, and the contact member is opposite to the direction in which the fixing nip portion protrudes at the central portion in the longitudinal direction. Characterized in that it comprises a shape protruding.

  According to the present invention, it is possible to suppress variations in the longitudinal direction of the width of the fixing nip portion and to suppress toner adhesion to the trailing edge of the recording material in the transfer nip portion.

1 is a diagram illustrating an overall configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 3 is a diagram for explaining the shape of a recording material held in a fixing nip portion according to the first embodiment. The figure explaining the positional relationship with the fixing nip part of the fixing inlet guide of 1st Embodiment, and the shape of a fixing inlet guide. The figure explaining the 1st effect by using the fixing entrance guide of a 1st embodiment. The figure explaining the said 1st effect compared with a prior art example The figure explaining the 2nd effect by using the fixing entrance guide of a 1st embodiment. The figure which compared the structure of the fixing entrance guide which concerns on 2nd Embodiment with the structure of the fixing entrance guide which concerns on 1st Embodiment. The figure explaining the effect in 2nd Embodiment The figure explaining the positional relationship with the fixing nip part of the fixing inlet guide of 2nd Embodiment, and the shape of a fixing inlet guide. FIG. 6 is a diagram for explaining that a thin recording material is conveyed to a fixing nip portion using the fixing inlet guide according to the second embodiment. The figure explaining the positional relationship with the fixing nip part of the fixing inlet guide of 3rd Embodiment, and the shape of a fixing inlet guide. FIG. 6 is a diagram for explaining the occurrence of trailing edge contamination in a comparative image forming apparatus (fixing nip portion is flat in the longitudinal direction). FIG. 6 is a diagram for explaining the occurrence of trailing edge contamination in a conventional image forming apparatus (the fixing nip portion is convex in the longitudinal direction toward the pressure roller). FIG. 6 is a diagram for explaining an angle θ of an image forming apparatus having a general C path configuration.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

<< First Embodiment >>
(Image forming device)
An overall configuration of an image forming apparatus according to an embodiment of the present invention will be described with reference to FIG. The configuration is the same as that of an image forming apparatus having a general C pass configuration except for the shape of a fixing entrance guide described later.

  The image forming apparatus shown in FIG. 1 includes a drum-type electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) 1 as an image carrier. The photosensitive drum 1 is rotatably supported by the apparatus main body 100 and is rotationally driven by a driving means (not shown) at a process speed of about 170 mm / sec (30 sheets / min with LTR size paper) in the direction of the arrow Ra.

  Around the photosensitive drum 1, a charging roller (charging device) 2, an exposure unit 3, a developing device 4, and a transfer roller (transfer device) 5 are arranged in this order along the rotation direction Ra. Here, the photosensitive drum 1 and the transfer roller 5 function as a transfer member that forms the transfer nip portion Nt.

  A paper cassette 11 containing a transfer material P as a recording material (paper) is disposed at the lower part of the apparatus main body 100. The paper feed roller 12, the conveyance guide 28, the fixing inlet guide (inlet guide) 271 according to the present embodiment, the fixing device (fixing unit) 6, and the paper discharge are sequentially arranged along the conveyance path (arrow in the figure) of the paper P. A roller 14 is disposed.

  The photosensitive drum 1 that is rotationally driven in the direction of the arrow Ra by a drive unit (not shown) is uniformly charged to a predetermined polarity and a predetermined potential by the charging roller 2. The photosensitive drum 1 after charging is subjected to image exposure based on the image information by the exposure means 3 such as a laser optical system on the surface, and the charge of the exposed portion is removed to form an electrostatic latent image. The electrostatic latent image is developed by the developing device 4. The developing device 4 has a developing roller 41, and a developing bias is applied to the developing roller 41 to attach toner to the electrostatic latent image on the photosensitive drum 1, thereby developing (visualizing) the toner image (toner image). To do.

  The toner image is transferred onto the paper P by the transfer roller 5. The transfer material P is stored in the paper cassette 11, fed by the paper feed roller 12, and conveyed along the conveyance guide 28 to the transfer nip Nt portion between the photosensitive drum 1 and the transfer roller 5. A transfer bias is applied to the transfer roller 5, whereby the toner image on the photosensitive drum 1 is transferred to a predetermined position on the paper P.

  The sheet P carrying the unfixed toner image on the surface by the transfer is fixed by the fixing device 6 along the fixing inlet guide 271 as a contact member (guide member) that contacts the surface opposite to the surface on which the toner image is provided. It is conveyed to the nip portion Nf. At the fixing nip portion Nf, the unfixed toner image is heated and pressurized and fixed on the surface of the paper P. The paper P after the toner image is fixed is discharged onto a paper discharge tray 15 on the upper surface of the apparatus main body 100 by a paper discharge roller 14.

  On the other hand, the photosensitive drum 1 after the transfer of the toner image is again charged by the charging roller 2 and used for the next image formation. By repeating the above operation, image formation can be performed one after another.

  In the image forming apparatus of the present embodiment, the distance from the central position in the recording material conveyance direction of the transfer nip portion Nt to the central position in the recording material conveyance direction of the fixing nip portion Nf is defined as a transfer-fixing distance. The thickness was 50 mm. Further, the angle θ formed by the tangent line of the fixing nip portion Nf with respect to the tangent line of the transfer nip portion Nt was set to 30 °.

(Fixing device)
In the specification of the present application, in the fixing nip portion or the like, the central portion in the longitudinal direction, which is a direction orthogonal to the recording material conveyance direction, is a central uniform region or a predetermined region in the region when equally divided into three in the longitudinal direction. Position. Further, the end portion in the longitudinal direction is defined as an equal region on the end portion side or a predetermined position in the region (for example, a position on the most end portion side) when equally dividing into three equal parts in the longitudinal direction.

  The fixing device 6 of this embodiment shown in FIGS. 1 and 2 is a film heating type fixing device for the purpose of shortening the start-up time and reducing power consumption. A cylindrical fixing film 24 serving as a rotating body is a flexible endless belt that can be rotated. A ceramic heater 242 serving as a heating member, a heater holder 241 serving as a backup member, and an iron stay 243 are disposed on the inner periphery thereof. Is provided.

  The heater holder 241 holds a ceramic heater 242 and contacts the inner surface of the fixing film 24 to heat the fixing film 24 from the inside. A fixing nip portion Nf is formed by the ceramic heater 242 and the pressure roller 23 as an opposing member facing the ceramic heater 242 through the fixing film 24 so that the fixing film 24 is nipped and conveyed. Further, since the heater holder 241 is made of heat-resistant resin, the heater holder 241 does not have high rigidity and is reinforced by a steel stay 243 from a surface opposite to the ceramic heater 242 holding surface.

  The pressure roller 23 is driven by a driving source, and the fixing film 24 receives power from the pressure roller 23 at the fixing nip portion Nf and rotates following the power. The heat of the heater 242 is transferred from the inner surface of the fixing film 24 to the surface, and the surface of the pressure roller 23 is also heated at the fixing nip portion Nf. As described above, when the paper P to which the unfixed toner image is transferred is conveyed to the fixing nip Nf, the heat of the fixing film 24 and the pressure roller 23 is transmitted to the unfixed toner image and the paper P, and the paper A toner image is fixed to P.

  The heater holder 241 used in the present embodiment is smoothly convex in the same direction as the pressure roller 23 that is bent by being urged by the heater holder 241 and both ends from both ends in the longitudinal direction to the center. It is formed in a curved cross section. The amount of bending of the pressure contact surface of the heater holder 241 with the pressure roller 23 is such that the central portion in the longitudinal direction with respect to both end positions of the maximum sheet width is curved 520 μm toward the urging direction pressure roller 23 side.

  The pressure roller 23 is provided with a heat-resistant elastic layer having elasticity such as silicone rubber on the outer peripheral surface of a metal core and a release layer having release properties such as fluororesin on the outermost surface. Is. As the pressure roller 23 used in this embodiment, a core metal outer diameter of Φ11 mm, an elastic layer portion having an outer diameter of Φ18 mm, and a straight one in the longitudinal direction were used.

  The surface hardness of the elastic layer portion including the release layer is 40 ° in Asker-C hardness (500 g load). The biasing force of the heater holder 241 to the pressure roller 23 was 13 kgf. Since the heater holder 241 is formed in a curved cross section that is convex in the same direction as the pressure roller 23, the width of the fixing nip portion Nf in the conveyance direction is about 7 mm and substantially uniform in the longitudinal direction.

(Curved shape of recording material following curved shape of fixing nip Nf)
As described above, the fixing nip portion Nf of the fixing device 6 is curved toward the pressure roller side in the urging direction of the pressure roller 23 and the heater holder 241 with the central portion in the longitudinal direction as viewed from the recording material conveyance direction. It has a shape.

  Actually, when the fixing device 6 is removed from the image forming apparatus, and the sheet P is engaged only in the fixing nip portion Nf, the sheet P has the same shape as the fixing nip portion Nf as shown in FIG. The shape is regulated to a shape curved toward the non-printing surface side of the paper P with the central portion in the paper width direction as a substantial vertex. In the present embodiment, the amount of bending of the central portion toward the non-printing surface side with respect to both ends in the longitudinal direction (paper width direction) of the paper P was 500 μm.

(Fixing entrance guide)
The fixing inlet guide 271 in this embodiment is a guide for guiding the paper P that has passed the transfer nip portion Nt and holds the toner image to the fixing nip portion Nf of the fixing device 6 that is the next step. Hereinafter, the fixing entrance guide 271 according to the present invention will be described.

  The fixing inlet guide 271 of the present embodiment has a convex portion M that protrudes in the direction opposite to the curve direction of the fixing nip portion Nf at the center in the longitudinal direction. And the convex part M is provided in the position which can lift the width direction center part of the paper P curved to the non-printing surface side mentioned above to the printing surface side direction. Specifically, as shown in FIG. 7A, the convex portion M is provided as a partial region in the entire region of the fixing entrance guide 271.

Here, parameters for explaining the installation position of the fixing entrance guide 271 are defined with reference to FIG. FIG. 3A is a schematic cross-sectional view of the fixing device 6 and the fixing inlet guide 271. FIG. 3B shows the relationship between the fixing inlet guide 271 and the fixing nip portion Nf when viewed from the arrow A direction (the upstream side in the sheet P conveying direction on the fixing nip tangent) which is the recording material conveying direction in FIG. It is a figure which shows a positional relationship. S: downstream end of conveyance direction of fixing inlet guide 271 Hnipc: biasing direction position (reference position) at the center in the longitudinal direction of fixing nip Nf
Hnipe: urging direction position at both longitudinal ends of the fixing nip portion Nf Hgc: urging direction position at the longitudinal center of the tip S of the fixing inlet guide 271 Hge: attachment at the longitudinal end of the tip S of the fixing inlet guide 271 Force direction position ΔHm: Projection amount of the portion (convex portion M) where the projection amount from the reference position to the heater holder side is maximum in the recording material conveyance direction (ΔHm = Hgc−Hge)
Hereinafter, the position of the fixing entrance guide 271 and the protrusion amount ΔHm of the protrusion M will be described using the defined parameters. The position of the fixing entrance guide 271 is the position Hnipc in the urging direction of the fixing nip portion Nf over the entire longitudinal direction so that no jamming or paper wrinkle occurs when the leading edge of the paper P enters the fixing nip portion Nf. And it is necessary to be installed at a position not too far from Hnipe. In the present embodiment, in the fixing inlet guide 271, the urging direction position Hge of the tip S at both ends in the longitudinal direction is the same as the urging direction position Hnipc of the fixing nip portion Nf at the center in the longitudinal direction that is the reference position. Installed in position.

  If the protrusion amount ΔHm of the convex portion M is too large, the leading edge of the paper P is transported to the biasing direction heater holder side rather than the fixing nip portion Nf, and the unfixed toner image on the paper P is flexible. In some cases, an “image disorder” is disturbed by hitting the fixing film. On the other hand, if it is too small, the ability to lift the central portion in the width direction of the paper P curved toward the non-printing surface cannot be exhibited. Therefore, the protrusion amount ΔHm of the protrusion M is preferably about 50 to 2000 μm.

  The size of the fixing entrance guide 271 in the longitudinal direction (width direction) (La in FIG. 3) is preferably approximately the maximum sheet width. This is because if the size is too large, the device becomes huge. On the other hand, if the width is too small with respect to the maximum sheet width, the height of the leading end of the sheet P at both ends of the maximum sheet is too low with respect to the fixing nip portion Nf, especially for thin paper with weak stiffness, and the leading end of the sheet P In some cases, corner breakage or the like may occur without smoothly entering Nf. In this embodiment, the overall width La is 218 mm with respect to the width 215 mm of the LTR size paper which is the maximum paper.

  The width Lm of the convex portion M of the fixing entrance guide 271 only needs to be wide enough to lift the shape of the trailing edge of the sheet P curved toward the non-printing surface at the transfer nip portion Nt. However, if the width Lm of the convex portion M becomes too large, the paper may not enter smoothly into the fixing nip portion Nf, particularly with strong paper such as thick paper, and the above-described image disturbance may occur. There is a case to do.

  This is because the larger the width Lm of the convex portion M, the larger the amount by which the leading end of the sheet P at the center in the longitudinal direction is lifted to the heater holder side even if the protrusion amount ΔHm is set to be the same as when the width Lm of the convex portion M is narrow. It means to become. That is, the gap with the fixing nip Nf that is curved toward the pressure roller side is widened. Therefore, a ratio of 1/5 to 3/5 with respect to the width La of the entire area in the longitudinal direction of the fixing entrance guide is preferable, and in this embodiment, the width is set to 80 mm.

As the material of the fixing entrance guide 271, a material in which a conductive material such as carbon is mixed with PET and the resistance value is adjusted to about 1 × 10 ¹² Ωcm is used. The reason is that the fixing entrance guide 271 disturbs the toner image on the unfixed sheet P by contacting the sheet P holding the charge with the transfer bias, or by removing the charge of the sheet P. Is to prevent.

(Evaluation method)
Hereinafter, the evaluation method of this embodiment will be described. The fixing inlet guide 271 having the convex portion M of the present embodiment and the conventional fixing inlet guide were checked for the presence or absence of trailing edge contamination. As an evaluation condition, a single sheet passing was repeated 500 times in a high temperature and high humidity environment where the “fogging” was severe temperature of 35 ° C. and humidity of 80%, and the toner adhesion state on the rear end of the output paper P was confirmed. Thereby, it is possible to compare the degree of fog toner accumulated on the transfer roller 5 being scraped off by the trailing edge of the paper P and adhering to the trailing edge of the paper P.

  The protrusion amount ΔHm of the convex portion M provided on the fixing inlet guide 271 to the positive direction side (heater holder side) of the biasing direction was selected within a range in which the unfixed toner image is not disturbed. Specifically, the protrusion amount ΔHm of the convex portion M was confirmed for each level of 100 μm, 300 μm, 500 μm, and 1000 μm.

  Further, in order to confirm the influence of the distance between the transfer and the fixing, the distance between the transfer and the fixing is 50 mm (the distance between the transfer and the fixing in the present embodiment), and also when the distance is 80 mm, 100 mm, and 150 mm. confirmed. In these evaluations, the distance between the transfer and fixing is increased by changing only the height of the main body without changing the angle θ formed between the tangent line of the transfer nip portion Nt and the tangent line of the fixing nip portion Nf.

The case where the toner adhesion at the rear end of the paper P was conspicuous was evaluated as x, and the case where the toner adhesion at the rear end of the paper P was slight or the toner adhesion at the rear end of the paper P was not observed was evaluated as ◯. The paper used was Xerox® Vitality ^™ Multipurpose Printer Paper 75g.

(Evaluation results)
Table 1 shows the evaluation results of the present embodiment (results of comparing the trailing edge edge dirt level of the conventional fixing inlet guide and the fixing inlet guide of the present embodiment).

  In the conventional configuration using the conventional fixing entrance guide not provided with the convex portion M, the rear end edge stains become conspicuous when the distance between the transfer and fixing of the main body is shortened. This is presumably because the trailing edge of the paper P becomes noticeable due to the bending rigidity of the paper P because the trailing edge of the paper P can easily scrape the fog toner accumulated on the transfer roller 5. When the transfer-fixing distance was as large as 150 mm, no trailing edge staining occurred.

  On the other hand, in the configuration of the present embodiment using the fixing entrance guide 271 provided with the convex portion M, the rear end edge stains become smaller as the protrusion amount ΔHm of the convex portion M increases, even in a shorter transfer-fixing distance configuration. I was able to suppress it.

(Effect of this embodiment)
Two effects that can suppress the trailing edge contamination by using the fixing inlet guide 271 of this embodiment will be described below.

1) First effect The first effect is that the shape of the trailing edge of the sheet P can be changed so that the trailing edge of the sheet P hits the transfer roller 5 over the entire area in the longitudinal direction (width direction). It is. This point will be described with reference to FIGS. FIG. 4 is a schematic diagram of the transfer nip portion Nt and the fixing nip portion Nf in the image forming apparatus 100 of the present embodiment. FIG. 12 and FIG. 13 showing the transfer nip portion Nt and the fixing nip portion Nf of the conventional configuration are shown.

  FIG. 4A shows the shape of the paper P at the moment when the trailing edge of the paper P passes through the transfer nip portion Nt. FIG. 4B shows a state where the trailing edge of the rear end of the paper P is stained, and a portion where the trailing edge is stained is indicated by a hatched portion.

  In the conventional fixing entrance guide without the convex portion M, as shown in FIG. 13B, the trailing edge of the sheet P at the moment of passing through the transfer nip portion Nt is on the negative side of the urging direction of the fixing nip portion Nf ( The curved shape on the pressure roller side and the cylindrical shape of the transfer roller collided and were in a state of undulation. For this reason, the contact pressure is concentrated only in a predetermined area (shaded area in FIG. 13A) in contact with the transfer roller 5.

  However, when the fixing entrance guide 271 that protrudes in the center portion in the longitudinal direction opposite to the direction in which the fixing nip portion protrudes is used as in the present embodiment, as shown in FIG. The center part in the longitudinal direction (width direction) is lifted. For this reason, the shape of the trailing edge of the sheet P at the moment of passing through the transfer nip Nt is regulated to be almost flat. The rear end of the sheet P that has become substantially flat comes into contact with the transfer roller 5 in the entire width direction of the sheet P.

  Therefore, the pressure of contact between the rear end of the paper P and the surface of the transfer roller 5 can be distributed over the entire width direction of the paper P, and the toner hardly adheres to the rear end of the paper P at each position in the width direction of the rear end of the paper P. I was able to.

2) Second effect The second effect is that the restoring force of the paper P at the moment when the trailing edge of the paper P passes through the transfer nip portion Nt can be reduced. This point will be described with reference to FIG. FIG. 3 is a schematic cross-sectional view of the image forming apparatus showing a member constituting the transfer nip portion Nt and the fixing nip portion Nf of the fixing device 6 and the fixing inlet guide 271. The convex portion M of the fixing inlet guide 271 at the central portion in the longitudinal direction, which is a feature of this embodiment, is a knitting portion. With respect to the path of the pressure roller 23 and the paper P, the central portion is indicated by a solid line, and the end portion is indicated by a dotted line.

  When the conventional fixing entrance guide without the convex portion M is used, at the moment when the trailing edge of the paper P passes through the transfer nip portion Nt, the force that eliminates the paper deflection by the angle θ is applied to the surface of the transfer roller 5 and the paper P. It was pressure contact with the rear end. However, when the fixing entrance guide having the convex portion M at the central portion in the longitudinal direction is used, the force (that is, the restoring force) for eliminating the bending of the paper at the moment when the rear end of the paper P passes through the transfer nip is the angle θ ′. Decrease in minutes. Therefore, the pressure contact force between the surface of the transfer roller 5 and the rear end of the paper P can be reduced, and the toner on the surface of the transfer roller 5 hardly adheres to the rear end of the paper P.

  Next, an effect comparison was made between the present embodiment and a configuration (comparative example) in which the fixing inlet guide is not provided with the convex portion M and the fixing inlet guide is mounted on the biasing direction heater holder 241 side. About this comparative example, it becomes possible to make angle (theta) small like this embodiment. However, when the installation position of the conventional fixing entrance guide without the convex portion M is changed, the rear end edge contamination is tried to be confirmed by the same evaluation method. However, when the entire fixing entrance guide is changed to the heater holder 241 side. A jam has occurred. This is because the front end of the paper P is separated from the fixing nip portion Nf toward the heater holder 241 in the entire longitudinal direction.

  On the other hand, in the present embodiment in which the convex portion M is provided, the front end of the sheet P is separated from the fixing nip portion Nf toward the heater holder only in the vicinity of the center portion in the longitudinal direction, and both end portions in the longitudinal direction are directed to the pressure roller 23 side. approach. For this reason, the paper P can enter the fixing nip portion Nf in order from both ends in the longitudinal direction, and jamming does not occur and there is no problem in transportability.

  As described above, the fixing inlet guide 271 having the convex portion M opposite to the curve direction of the fixing nip portion is used to suppress the variation in the longitudinal direction of the width of the fixing nip portion due to the curve of the fixing nip portion, and at the transfer nip portion. It is possible to suppress toner adhesion to the rear end of the recording material (rear end edge contamination). Even in the case of an image forming apparatus having a short transfer-fixing distance (C pass) as the recording material conveyance path, the contact pressure between the rear end of the sheet P and the surface of the transfer roller 5 can be reduced. Rear end edge contamination can be suppressed.

<< Second Embodiment >>
Next, in the case where the recording material is a weak paper such as thin paper, a second embodiment in which the first embodiment is further improved by suppressing the corner bending (detailed later) that tends to occur. Will be described with reference to FIGS. This embodiment is different from the first embodiment only in the fixing entrance guide as shown in FIG. 7, and the other configurations are the same as those in the first embodiment, and thus the description thereof is omitted.

  FIG. 7A shows the fixing inlet guide 271 in the first embodiment, while FIG. 7B shows the fixing inlet guide 272 in the present embodiment. In other words, in the first embodiment, the fixing inlet guide 271 is located at the center portion in the longitudinal direction closest to the fixing nip portion and the distance between the fixing nip portion at the both ends in the longitudinal direction. And the fixing nip portion.

  On the other hand, the fixing inlet guide 272 of the present embodiment is such that the distance between the fixing nip and the position closest to the fixing nip at the center in the longitudinal direction is the closest to the fixing nip at both ends in the longitudinal direction. It is longer than the distance to the fixing nip.

(Folded corners of thin paper)
Hereinafter, the corner break at the leading edge of the thin paper will be described with reference to FIG. FIG. 8 shows a state when the LTR size paper P having the maximum paper passing width is passed, and FIG. 8A shows a state after the front end of the paper P passes the front end S of the fixing inlet guide. 8B shows a state immediately before entering the fixing nip portion Nf. In FIG. 8B, the fixing inlet guide 272 shows a cut state at the Sx position in FIG. 7B.

  In order to make it easy to understand the behavior of both ends of the paper P, the cross-sectional portions at the positions of both ends in the longitudinal direction (width direction) of the paper are indicated by solid lines, and the cross-sectional portions at the center are indicated by dotted lines. Further, the convex portion M at the center in the paper width direction is a knitted portion in the figure.

  In FIG. 8A, the front end of the central portion in the longitudinal direction (width direction) of the paper P has a shape that is lifted from both ends by the convex portion M of the fixing inlet guide 271. On the other hand, both end portions in the longitudinal direction (width direction) of the paper P are likely to fall in a direction opposite to the direction lifted at the center portion. The direction lifted at the center is the urging direction heater holder 241 side, and the direction of depression of both ends is the urging direction pressure roller side.

  If the amount of depression at both ends in the longitudinal direction (width direction) of the paper P toward the urging direction pressure roller is large, the paper P immediately before entering the fixing nip portion Nf as shown in FIG. 8B. The corner of P breaks (O portion in the figure). If the sheet enters the fixing nip portion Nf as it is, the sheet P may be discharged in a state where the sheet P is broken.

(Fixing entrance guide)
Hereinafter, the fixing inlet guide 272 of the present embodiment will be described with reference to FIGS. 7B and 9. The fixing inlet guide 272 extends the leading end of the longitudinal end region U of the fixing inlet guide of the first embodiment to Su in the drawing toward the fixing nip portion Nf. That is, the distance s from the center line (line Z in FIG. 9) of the fixing nip portion Nf in the conveyance direction of the sheet P at the front end S in the longitudinal center is longer than the distance su from the front end Su at both ends in the longitudinal direction. Hereinafter, the difference between the distance s and the distance su is referred to as ΔS, and is referred to as the length ΔS of the U portion provided in the fixing inlet guide 272.

  If the length ΔS of the U portion of the fixing entrance guide 272 is too long, it may hit the pressure roller 23 or the fixing film 24 of the fixing device 6, and if it is short, the function of preventing the leading edge of the paper P from falling cannot be exhibited. You may set to length (DELTA) S which can exhibit an effect by structure. In the present embodiment, the length ΔS of the U portion is 2 mm.

  The width Lu of the U portion only needs to have a width that can prevent the both ends of the paper P in the width direction from dropping. However, if the width Lu of the U portion is too large and there is a portion in the central region in the longitudinal direction that approaches the fixing nip portion Nf, the portion strongly rubs the non-printing surface of the paper P and disturbs the unfixed toner. Or streaks may occur on the paper P.

  When the sheet P in the central portion in the longitudinal direction enters the fixing nip portion Nf that is curved toward the pressure roller in the urging direction, the shape becomes a thick line Pc 'shown in FIG. If the leading end of the fixing entrance guide is on the locus of the thick line Pc ′, the leading end of the fixing entrance guide strongly rubs the non-printing surface of the paper. Therefore, in the present embodiment, the position of the tip S in the central region in the longitudinal direction remains Zs in the drawing, and only the both end regions are brought close to the fixing nip portion Nf up to Zsu in the drawing, and the width Lu of the U portion is 65 mm. . Moreover, the protrusion amount of the convex part M was 500 μm in the present embodiment, which was able to suppress the trailing edge edge contamination in the first embodiment.

(Evaluation method)
Hereinafter, the evaluation method of this embodiment will be described. The fixing entrance guide 272 having the convex portion M and the portion U extending in the fixing nip portion direction of the present embodiment, the fixing entrance guide 271 having the convex portion M used in the first embodiment, and the convex portion M are provided. With a conventional fixing entrance guide, the presence or absence of trailing edge contamination and the presence of corner breakage were confirmed.

As evaluation conditions, the evaluation of the trailing edge edge stain was performed under the same conditions as those described in the first embodiment. Regarding the evaluation of the corner breakage of the leading edge, in order to confirm the influence of the paper basis weight, the same evaluation as that of the trailing edge edge was performed using paper having different basis weights, and 52 g / cm ² , 60 g / cm ² and 75 g / cm ^2. , 90 g / cm ² was confirmed.

  As for the trailing edge, as in the first embodiment, the case where the dirt on the trailing edge of the paper P is noticeable is evaluated as x, and the case where the dirt on the trailing edge of the paper P is slight or not seen is evaluated as ◯. Regarding the tip corner breakage, the case where no corner breakage occurred was evaluated as ◯, and the case where corner breakage occurred was evaluated as ×.

(Evaluation results)
Table 2 shows a comparison result of the trailing edge edge dirt level and the tip corner breakage.

In the configuration using the conventional fixing entrance guide 270, the front end corner breakage does not occur, but the rear end edge contamination occurs. In addition, when the shape of the fixing entrance guide 271 of the first embodiment is used, it is possible to prevent the occurrence of trailing edge contamination, and the leading edge breakage occurs in a sheet having a basis weight of 75 g / cm ² or more. I did not.

  On the other hand, in the configuration using the fixing entrance guide 272 having the convex portion M and the portion U extending in the fixing nip direction as in the present embodiment, both the rear end edge contamination and the leading end corner bending phenomenon can be suppressed. It was. This is because, even in the case of thin paper and weak paper, it is possible to suppress the drop in both ends in the width direction of the paper P by the extending portions U provided in the both end regions of the fixing entrance guide 272.

(Effect of this embodiment)
As described above, the fixing inlet guide 271 having the convex portion M opposite to the curve direction of the fixing nip portion is used to suppress the variation in the longitudinal direction of the width of the fixing nip portion due to the curve of the fixing nip portion, and at the transfer nip portion. It is possible to suppress toner adhesion to the rear end of the recording material (rear end edge contamination). Even in the case of an image forming apparatus having a short transfer-fixing distance (C pass) as the recording material conveyance path, the contact pressure between the rear end of the sheet P and the surface of the transfer roller 5 can be reduced. Rear end edge contamination can be suppressed.

  Furthermore, it is possible to suppress the corner breakage of the leading edge of the paper that is likely to occur with a soft paper such as thin paper. This is because both ends in the longitudinal direction (width direction) of the leading edge of the sheet have an effect of preventing the sheet from being easily dropped in a direction opposite to the direction lifted at the center.

<< Third Embodiment >>
In the first and second embodiments, a configuration in which the posture of the sheet P in a state where the sheet P passes through the transfer nip portion Nt and is held only by the fixing nip portion Nf follows the tangent line of the fixing nip portion Nf will be described. Did. In the present embodiment, the position of the paper P in a state where the paper P passes through the transfer nip portion Nt and is held only by the fixing nip portion Nf is different from the above-described embodiment in that it is not tangent to the fixing nip portion Nf. . That is, in the present embodiment, the posture of the sheet P in a state where the sheet P passes through the transfer nip portion Nt and is held only by the fixing nip portion Nf is the upstream protruding shape portion (upstream protruding portion) provided in the heater holder 241. Part) Follow G.

  In addition, since components and configurations other than the heater holder 241 and the evaluation method are the same as those described in the second embodiment, description thereof will be omitted.

  FIG. 11 is a schematic cross-sectional view of the transfer nip portion Nt, the fixing device 6, and the fixing inlet guide in the image forming apparatus used in this embodiment. In general, the heater holder 241 may have a shape in which the upstream protruding shape portion G protrudes about several hundred μm from the surface where the heater 242 contacts the inner surface of the fixing film 24. The purpose of this is to prevent the fixing film 24 from being torn when a sheet with staples enters. As the upstream protruding shape portion G of the heater holder 241 of the present embodiment, one that protrudes 200 μm in the urging direction with respect to the surface where the heater 242 contacts the inner surface of the fixing film 24 is used.

(An angle θ2 in this embodiment)
As described above, when the angle θ formed by the tangent line of the transfer nip portion Nt and the posture of the paper P at the moment when the rear end of the paper P passes through the transfer nip portion increases, Since the contact pressure with the roller surface increases, the trailing edge edge contamination increases.

  Below, angle (theta) 2 in this embodiment using the said heater holder 241 is demonstrated. In the present embodiment, the upstream protruding shape portion G of the heater holder 241 causes the paper P to have a shape restricted by the restriction portion g in the drawing when the rear end of the paper P passes through the transfer nip portion Nt. That is, at the moment of passing through the transfer nip portion Nt, the trailing edge of the paper P tries to follow not the tangent line of the fixing nip portion Nf but the straight line Y connecting the central portion of the fixing nip portion Nf with respect to the paper P conveyance direction and the regulating portion g. .

  The angle θ2 formed by the straight line Y and the tangent line of the transfer nip portion Nt is larger than the angle θ1 formed by the tangent line of the transfer nip portion Nt and the tangent line of the fixing nip portion Nf, and the specific angle θ2 is 45 °. . Such an angle θ2 is stricter as a condition of rear end edge contamination than in the embodiment described above, but a fixing entrance guide 272 having a convex portion M and a portion U extending in the fixing nip portion direction is used. Thus, the same effect as that shown in the second embodiment can be obtained.

(Modification)
As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

(Modification 1)
In the above-described embodiment, the configuration in which the cross-sectional shape of the heater holder as the backup member is curved as the means for curving the shape of the fixing nip portion has been described, but the configuration is not limited thereto. Any means may be used as long as the fixing nip portion is bent toward the pressure roller. For example, in order to prevent paper wrinkling, the outer diameter of the pressure roller is larger at both ends than the longitudinal central portion. A configuration in which the portion has a larger shape may be used.

(Modification 2)
In the above-described embodiment, the shape of the fixing entrance guide has been described with respect to the rectangular configuration provided with the convex portion protruding in the opposite direction to the curved shape of the fixing nip portion, but the configuration is not limited thereto. For example, a smoothly curved shape or a shape in which the protruding amount gradually increases stepwise from both ends in the longitudinal direction to the center portion may be used.

  Further, the fixing entrance guide is generally shaped without a corner. This is because the fixing entrance guide changes the transport path at the leading edge of the sheet and guides it to the fixing device 6, and contacts the non-printing surface of the sheet, thereby preventing streaks on the sheet. It is. It is more preferable that the convex portion M of the fixing inlet guide and the portion U extending in the fixing nip direction described in the present embodiment are continuously (smoothly) inclined and connected to other portions.

(Modification 3)
In the embodiment and the modification described above, the inlet guide that is long in the longitudinal direction has been described, but the present invention is not limited to this. That is, when equally dividing into three equal parts in the longitudinal direction orthogonal to the recording material conveyance direction, the contact member has a spot-like shape (a point that protrudes toward the heating member in the central uniform region in the longitudinal direction). A) contact member.

  Thus, a conventional fixing inlet guide (having a shape with a uniform height in the longitudinal direction) and the spot-shaped contact member may be provided side by side in the recording material conveyance path. Alternatively, a conventional fixing inlet guide (having a shape with a uniform height in the longitudinal direction) provided with the spot-like contact member and integrated is used as the fixing inlet guide in the above-described embodiment. May be.

1 .... photosensitive drum, 5 .... transfer roller, 23..pressure roller, 242..heater, 271,272..fixing inlet guide, Nf..fixing nip, Nt..transfer nip.

Claims (15)

A transfer member that forms a transfer nip for transferring a toner image to a recording material;
A heating member for heating the recording material onto which the toner image has been transferred;
A facing member that forms a fixing nip that faces the heating member and sandwiches and conveys the recording material together with the heating member;
An abutting member provided in a recording material conveyance path between the transfer nip portion and the fixing nip portion and in contact with a surface of the recording material opposite to a surface on which the toner image is provided;
Have
When viewed from the conveyance direction of the recording material, the fixing nip portion has a shape in which a central portion protrudes toward the opposing member from an end portion in a longitudinal direction orthogonal to the conveyance direction of the recording material, and The image forming apparatus according to claim 1, wherein the contact member has a shape protruding at a central portion in the longitudinal direction opposite to a direction in which the fixing nip protrudes.   The image forming apparatus according to claim 1, wherein the contact member is a guide member provided over the longitudinal direction for guiding the recording material to the fixing nip portion.   The image forming apparatus according to claim 2, wherein the guide member is provided with a region protruding in a direction opposite to a protruding direction of the fixing nip portion as a partial region in the entire region of the guide member.   2. The guide member according to claim 1, wherein a region protruding in a direction opposite to a protruding direction of the fixing nip portion is provided in a ratio of 1/5 to 3/5 with respect to the entire region in the longitudinal direction. The image forming apparatus according to 3.   The guide member has a distance between a position closest to the fixing nip portion at a central portion in the longitudinal direction and a center portion of the fixing nip portion in the recording material conveyance direction to the fixing nip portion at both end portions in the longitudinal direction. 5. The image forming apparatus according to claim 2, wherein the image forming apparatus is equal to a distance between a closest position and a central portion of the fixing nip portion in the recording material conveyance direction.   The guide member has a distance between a position closest to the fixing nip portion at a central portion in the longitudinal direction and a center portion of the fixing nip portion in the recording material conveyance direction to the fixing nip portion at both end portions in the longitudinal direction. 5. The image forming apparatus according to claim 2, wherein the image forming apparatus is longer than a distance between a closest position and a center portion of the fixing nip portion in the recording material conveyance direction.   The image forming apparatus according to claim 1, wherein the recording material conveyance path from the transfer nip portion to the fixing nip portion is provided in a C shape.   The image forming apparatus according to claim 7, wherein the recording material conveyance path includes a conveyance path in which a rear end of the recording material that has passed through the transfer nip portion follows a tangent to the fixing nip portion. A cylindrical rotating body heated by the heating member, and a backup member provided in the longitudinal direction and holding the rotating body,
The recording material conveyance path includes a central portion in the recording material conveyance direction of the fixing nip portion of the recording material that has passed through the transfer nip portion, and a restriction portion that is regulated by an upstream protruding portion of the backup member. The image forming apparatus according to claim 7, further comprising a conveyance path that follows a straight line connecting the two.   The said guide member is provided with the shape which inclined continuously toward the edge part of the said longitudinal direction from the area | region which protrudes on the opposite side to the direction where the said fixing nip part protrudes. The image forming apparatus according to claim 1. A cylindrical rotating body that is flexible and heated by the heating member; and a backup member that is provided in the longitudinal direction and holds the rotating body;
11. The image forming apparatus according to claim 1, wherein the backup member is pressed toward the opposing member at both ends in the longitudinal direction.   The image forming apparatus according to claim 11, wherein the backup member has a shape in which a central portion protrudes toward the facing member rather than an end portion in the longitudinal direction when viewed from the conveyance direction of the recording material. apparatus.   The image forming apparatus according to claim 1, wherein the facing member is a pressure roller.   The image forming apparatus according to claim 13, wherein the pressure roller has an outer diameter at both end portions larger than an outer diameter at a central portion in the longitudinal direction.   The transfer unit includes an image carrier that carries a toner image, and a transfer roller that transfers the toner image to the recording material, and the transfer nip portion is disposed between the image carrier and the transfer roller. The image forming apparatus according to claim 1, wherein the image forming apparatus is formed.