JP2005258111A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain stable printing by preventing gloss and fixability from decreasing in the axial middle of a fixing nip section, thereby making the gloss and fixability uniform. <P>SOLUTION: The fixing device includes: a heat roller 1; a pressure roller 2 which is pressed against the heat roller 1; a heat resistant belt 3 which is wound around the circumference of the pressure roller 2 and moved while held between the heat roller 1 and the pressure roller 2; and a belt stretch member 4 which stretches the heat resistant belt 3. The fixing device fixes an unfixed toner image formed on a sheet material 5. In the fixing device, the total nip width obtained by adding a nip between the heat roller 1 and the pressure roller 2 and a nip between the heat roller 1 and the belt stretch member 4 increases towards the axial middle from the ends. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention includes a heating roller, a pressure roller that presses against the heating roller, a heat-resistant belt that is attached to an outer periphery of the pressure roller and moves while being sandwiched between the heating roller, and the heat-resistant belt is stretched. The present invention relates to a fixing device that fixes a non-fixed toner image formed on a sheet material, and an image forming apparatus.

  In a heat roller type fixing device, a member such as a bearing is generally supported via both ends of a heating roller and a pressure roller, and either one of the rollers is configured to be able to swing, using springs or the like at both ends. Alternatively, as a structure for fixing both rollers, the toner is melted by applying a load by utilizing the elasticity of the elastic layer provided on the surface of the rollers and applying heat and pressure to the sheet material and the toner passing through the nip portion. Fixing.

  However, in such a structure, when a load is applied between the heating roller and the pressure roller, the roller bends, and as a result, the nip has a so-called inverted crown shape in which the central portion in the axial direction is narrower than the end portion. It is easy to become. In recent years, from the viewpoint of energy saving, it is a common practice to reduce the warm-up time by reducing the heat capacity by reducing the thickness and weight of members such as rollers. However, since such a small heat capacity roller has low rigidity, the nip-shaped reverse crown shape tends to become more prominent due to the deflection of the roller.

  On the other hand, the nip shape by the roller or the pressure member may be positively changed to an inverted crown shape (see, for example, Patent Document 1). If the nip shape is positively inverted crown shape, when the sheet material passes through the nip portion, a force that pulls the sheet material in both directions acts, so that generation of wrinkles can be prevented.

In addition, the belt nip type fixing device is advantageous in that the nip width can be increased compared to the roller method, the amount of heat transfer to the toner is increased, the speed is increased, and it is advantageous for fixing a color image for fixing a multilayer toner. It has been proposed (see, for example, Patent Document 2). Even in such a fixing device, particularly when a small heat capacity roller or a pressing member is used for shortening the warm-up time, the rigidity is lowered and the nip shape tends to be an inverted crown shape due to the bending.
JP-A-8-146806 Japanese Patent No. 3084692

  FIG. 10 is a diagram for explaining an example of forming the main nip, the auxiliary nip, and the gap between the nips. A conventional belt nip type fixing device (see, for example, Patent Document 2) further includes a pressure auxiliary roller in addition to a pressure roller. For example, a main nip and an auxiliary nip form a nip. In the case where the fixing nip is formed in this way, in the configuration of the main nip and the auxiliary nip, when the shape of the main nip by the roller is an inverted crown shape as shown in FIG. It is lower than the end. Therefore, the heat transfer efficiency to the toner in the nip portion is higher than that in the low pressure portion because the pressure and heat simultaneously act on the toner in the high pressure portion. In other words, the heat transfer efficiency is high at the axial end portion with a wide nip width, and the heat transfer efficiency is low at the axial center portion with a narrow nip width, so the amount of heat transfer to the toner is insufficient at the axial center portion more than the difference in nip width. It will be.

  On the other hand, a margin of several mm or more is usually formed at the end of the sheet material. Therefore, for example, by using a method such as changing the outer diameter shape of the roller in the axial direction, the amount of toner on the sheet can be set even when the pressure and nip shape are set to be uniform at the center and end in the axial direction. The center portion is larger than the axial end portion. In other words, since the heat capacity of the sheet on which the toner image is formed is larger in the central portion in the axial direction than in the end portion, the heat in the central portion in the axial direction is deprived of heat even when uniform pressure and heat are applied. There is a tendency that the temperature does not easily rise. As a result, there is a problem in that the gloss and fixability are lowered and non-uniform at the central portion in the axial direction and the image quality is lowered. This phenomenon is particularly noticeable in the case of a color image in which an image is formed by superposing a plurality of color toners, and the print quality is greatly impaired.

  The present invention solves the above-described conventional problems, and prevents a decrease in gloss and fixability at the central portion in the axial direction in the fixing nip, so that the gloss and fixability are uniform and stable printing can be obtained. It is to make.

  Therefore, the fixing device and the image forming apparatus according to the present invention are moved by being sandwiched between a heating roller, a pressure roller that presses against the heating roller, and an outer periphery of the pressure roller and sandwiched between the heating roller. A fixing device that fixes a non-fixed toner image formed on a sheet material, and is formed between the heating roller and the pressure roller. And the entire nip width formed by the nip formed between the heating roller and the belt stretching member is set to be larger than the end portion in the central portion in the axial direction.

  The heating roller has a plurality of heating sources, and the heat distribution by the plurality of heating sources is made larger at the central portion in the axial direction than at the end portion. It has a crown shape, and the pressure roller presses the heating roller with a predetermined pressing force, and the belt stretching member lightly presses the heating roller with a pressing force smaller than the pressure roller to nip the nip. The belt stretching member is configured to lightly press the heating roller by a frictional force with the heat-resistant belt when the heat-resistant belt is rotationally driven.

  The belt stretching member is configured to set a rotation fulcrum so as to be swingable in the rotation center direction of the heating roller, and to apply a tension in a tangential direction to a nip portion of the heating roller to tension the heating belt. It is constructed such that it is constructed so as to swing when paper is passed and press the heating roller, and is arranged on the upstream side in the sheet material conveying direction with respect to the pressure roller.

  The belt stretching member forms a nip by winding the heat-resistant belt around the heating roller with the pressure roller, and contacts the heating roller along the nip formed with the heating roller. The belt sliding member has a substantially half-moon shape and is formed so that the central portion has a high convex shape with respect to both end portions in the axial direction of the contact surface.

  According to the present invention, by making the entire nip width larger than the end portion in the axial center portion, the heat transfer amount to the toner is equal in the axial center portion and the end portion in the fixing nip, and heat transfer unevenness is eliminated. The amount of heat transfer to the toner in the central portion where the heat transfer efficiency to the toner is inferior to that in the end portion can be compensated, and the gloss and the fixing property are uniform in the axial center portion and the end portion, so that stable printing can be obtained. In addition, the reverse crown shape of the nip shape increases the peripheral speed of the edge compared to the center, and when the sheet material passes through the roller nip, pulls the sheet material in both directions to prevent wrinkling. can do. In addition, a gap is formed between the heat-resistant belt and the belt stretching member when the paper is not passed, and the amount of heat taken away from the heating roller via the heat-resistant belt during warming can be reduced by using the gap gap as a heat insulating layer. It is possible to reduce heat loss and warm-up time. On the other hand, when the sheet material passes through the fixing nip, the gap between the heat-resistant belt and the belt stretching member disappears, and the sheet material is pressed against the heat-resistant belt at the fixing nip and pressed against the heating roller, so that the stability is maintained. In this case, if the pressing force is adjusted to a desired pressure, proper fixing can be achieved.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating an embodiment of a fixing device according to the present invention, and FIG. 2 is a diagram illustrating the shape of a contact surface of a belt stretching member and the shapes of a stretching member nip and a roller nip. In the figure, 1 is a heating roller, 1a is a halogen lamp, 1b is a roller core, 1c is an elastic layer, 2 is a pressure roller, 2b is a roller core, 2c is an elastic layer, 3 is a heat resistant belt, 4 is a belt tension A support member, 4a is a protruding wall, 4-N is a heating roller contact surface, 5 is a sheet material, 5a is an unfixed toner image, and L is a pressing portion tangent.

  In FIG. 1, a heating roller 1 is, for example, a heating member having a built-in rotation of 1050 W as a heating source and two columnar halogen lamps 1a, and a pipe member having an outer diameter of about 25 mm and a wall thickness of about 0.7 mm. This is a heating-side rotary moving body formed by disposing a heating source inside the roller cored bar 1b and covering the outer periphery with an elastic layer 1c having a thickness of about 0.4 mm. As described above, since the heating roller 1 has a plurality of heating sources 1a built therein, when the heating elements of the halogen lamp are configured in different arrangements and selectively lit, a heat-resistant belt 3 to be described later is heated. The temperature under different conditions such as a fixing nip portion wound around 1 and a portion where the belt stretching member 4 slides on the heating roller 1 or a wide sheet material and a narrow sheet material. Control can be easily performed.

  The pressure roller 2 is a pressure member that is disposed opposite to the heating roller 1 and presses and urges to form a fixing nip. For example, the pressure roller 2 has an outer diameter of about 25 mm or 24.8 mm and is outside the roller core 1b of the heating roller 1. A pipe material having a small diameter and a thickness of about 0.7 mm is used as a roller core 2b, and the outer periphery thereof is covered with an elastic layer 2c having a thickness of about 0.2 to 0.3 mm that is thinner than the elastic layer of the heating roller 1. By forming the elastic layer, the rotating moving body on the other side has a harder surface than the elastic layer of the heating roller 1. As the pressure roller 2 having a hard surface, the surface whose surface is harder than the elastic layer of the heating roller 1 is a roller core metal or a roller core metal, and has an elastic layer on the surface. May be.

  The heat-resistant belt 3 is an endless belt which is sandwiched between the heating roller 1 and the pressure roller 2 and stretched around the outer periphery of the pressure roller 2 and the belt stretching member 4 so as to be movable. It has the above thickness. The material may be a heat resistant resin such as a polyimide tube or silicon, rubber, or the like, but a metal belt made of a stainless steel base material or a nickel electroformed tube excellent in thermal conductivity is preferable.

The belt stretching member 4 is disposed at a position where the heat-resistant belt 3 is wound around the heating roller 1 side from the pressing portion tangent L between the heating roller 1 and the pressure roller 2 to form a nip. The heating shown in FIG. 1B when the belt stretching member 4 is viewed from the side of the contact surface with the heating roller 1 with respect to the shape of the reverse crown-shaped roller nip formed between the heating roller 1 and the pressure roller 2. In the roller contact surface, as shown in the contact surface 4-N in FIGS. 2A and 2B, in the belt stretching member 4, the width W _NC at the center is larger than the width W _NE at the axial end. And formed into a crown shape. In addition, as shown in FIG. 2C, the total nip width including the width of the roller nip and the width of the stretching member nip is made larger than the end portion in the central portion in the axial direction. As a result, in the fixing nip, the amount of heat that is insufficient at the central portion in the axial direction is compensated, and the amount of heat transferred to the toner is equal at the central portion and the end portion in the axial direction. Thus, stable printing can be obtained.

  As described above, generally, the nip formed between the heating roller 1 and the pressure roller 2 tends to have an inverted crown shape. In this case, the amount of heat transferred to the toner at the central portion and the end portion in the axial direction is increased. Although there is a difference, by forming the belt stretching member 4 such that the nip width formed between the heating roller 1 and the belt stretching member 4 is larger at the center in the axial direction than at the end as described above. As shown in FIG. 2C, the nip has a so-called crown shape in which the central portion has a wider nip width than the end portion. As a result, the total nip width composed of the roller nip and the stretch member nip is made larger than the end portion in the axial center, so that the heat transfer efficiency is lowered due to the reverse crown shape of the roller nip, the toner image is formed, and the heat capacity of the sheet is reduced. The amount of heat that is insufficient at the central portion in the axial direction that becomes large can be compensated.

  In the fixing device according to this embodiment, the heat-resistant belt 3 is tensioned by the cooperation of the pressure roller 2 and the belt stretching member 4 to stretch the heat-resistant belt 3 and the heat-resistant belt 3 is wound around the heating roller 1. A fixing nip is formed, and the sheet material 5 on which the unfixed toner image 5a is formed is passed through the fixing nip to be fixed. The pressure contact force between the heating roller 1 and the pressure roller 2 is 10 kg or less, the nip width is about 10 mm as a whole, and is rotatable in the direction of the arrow shown.

  Originally, when the sheet material 5 on which the unfixed toner image 5a is formed is passed through the fixing nip and fixed, the sheet material 5 is imparted with a curling behavior that deforms along the outer periphery of the heating roller 1 and is wound around the heating roller 1 side. It will be easier. According to the present embodiment, since the outer diameters of the heating roller 1 and the pressure roller 2 are configured to be as small as about 25 mm, the sheet material 5 after fixing does not wrap around the heating roller 1 or the heat-resistant belt 3. Further, means such as a peeling claw and a peeling plate for forcibly peeling the sheet material 5 are made unnecessary.

  Further, in the pressure roller 2, if the outer diameter of the end portion in the axial direction is a reverse crown shape larger than the central portion, the peripheral speed of the end portion is faster than that of the central portion. When passing through the nip formed between the pressure roller 2 and the pressure roller 2, a pulling force acts in both end directions, and the generation of wrinkles can be prevented.

  Furthermore, when the pressure roller 2 having a harder surface than the elastic layer coated on the surface of the heating roller 1 is disposed opposite to the heating roller 1 and pressed and urged, the elastic layer of the heating roller 1 is elastically deformed and dented. A concave fixing nip is formed. When the heating roller 1 forms a concave fixing nip, the unfixed toner image formed on the sheet material is fixed and applied to the sheet material 5 that has passed through the fixing nip. And the elastic layer of the heating roller 1 is elastically deformed in the direction away from the heating roller 1 by the pressing of the pressure roller 2 that is harder than the elastic layer coated on the surface of the heating roller 1, and fixing. Curling behavior can be imparted to the opposite side of the heating roller 1 at the exit of the nip.

  In addition, the degree of curl behavior imparted to the sheet material 5 with respect to the moving direction of the heat-resistant belt 3 due to the hardness balance and pressing force of the pressure roller 2 and the heating roller 1 is not wound around the heating roller 1. It can be easily adjusted in a direction away from 1. As a result, it is possible to eliminate the peeling means for forcibly peeling the sheet material 5 after fixing without being wound around the heating roller 1 and the heat-resistant belt 3, and extremely stable image fixing is possible. Furthermore, if a PFA (peroxy-alkoxy-fluoro resin) layer of about 30 μm is provided on the surface layer of the elastic layer 1 c of the heating roller 1, the rigidity is improved by that much and the sheet material 5 and the toner are easily peeled. be able to.

  The heat-resistant belt 3 may be made of a rubber material such as PFA or silicon, for example, in consideration of the influence on the surface layer of the opposite heating roller 1 or the influence of contact with the toner image surface during double-side fixing to the sheet material 5. Good. When the heating roller 1 is heated to a predetermined temperature by warming up, the heat-resistant belt 3, the pressure roller 2, and the belt stretching member 4 are also increased to a predetermined temperature and stored. When a member having high thermal conductivity is used for the heat-resistant belt 3, the pressure roller 2, and the belt stretching member 4, the heat-resistant belt 3 is deprived of the sheet material 5 at the fixing nip by fixing the toner image on the sheet material 5. Since heat can be efficiently returned from the pressure roller 2 and the belt stretching member 4 to the heat-resistant belt 3, the temperature of the heat-resistant belt 3 can be lowered particularly when fixing is performed by passing a plurality of sheet materials 5 continuously. Can be suppressed.

  The belt stretching member 4 is disposed upstream of the nip portion between the heating roller 1 and the pressure roller 2 in the conveying direction of the sheet material 5, and for example, the heating roller 1 is centered on the rotation axis of the pressure roller 2. It is arranged so as to be swingable in the direction of the center of rotation. The belt stretching member 4 is configured to stretch the heat-resistant belt 3 in the tangential direction of the heating roller 1 at an initial position where the sheet material 5 enters the fixing nip when the sheet material 5 does not pass through the fixing nip.

  If the fixing pressure is large at the initial position where the sheet material 5 enters the fixing nip, the sheet material 5 may not enter smoothly and may be fixed with the leading edge of the sheet material broken. When the belt stretching member 4 is configured to stretch in the tangential direction of the heating roller 1 on the upstream side in the conveying direction of the sheet material 5, an introduction port portion that allows the sheet material 5 to enter smoothly can be formed, and a stable sheet can be formed. The material 5 can enter. In addition, if the fixing pressure is large at the end position where the sheet material 5 is discharged from the fixing nip, the sheet material 5 enters and is heated at the fixing nip, and finally the large fixing pressure is applied at the end position. Since the heating roller 1 side is concave and the curvature is large, the sheet material 5 is difficult to wind around the heating roller 1 and easily peels from the heating roller 1.

  If the belt stretching member 4 is structured such that it does not press the heating roller 1 when paper is not passed, but presses the heating roller 1 only when paper is passed, there is a gap between the belt 3 and the belt stretching member 4 when paper is not passed. The gap becomes a heat insulating layer and the amount of heat taken from the heating roller 1 via the belt 3 can be reduced, so that the warm-up time can be shortened. When the sheet material 5 passes through the fixing nip, the gap between the belt 3 and the belt stretching member 4 disappears, and the sheet material 5 is pressed against the belt 3 at the fixing nip portion and pressed against the heating roller 1. Therefore, stable fixing becomes possible. At this time, if the pressing force is adjusted to a desired pressure, proper fixing can be achieved.

  The belt stretching member 4 also has projecting walls 4a that project from both ends and are integrally formed. The protruding wall 4a has an abutting portion that abuts on the outer peripheral surface of the heating roller 1 outside the sheet passing region, and acts as a temperature leveling member that releases heat from both ends of the heating roller 1 to prevent an end temperature increase. Also, it acts as a gap adjusting member that adjusts the gap between the sliding surface of the belt stretching member 4 on which the heat-resistant belt 3 slides and the heating roller 1, or when the heat-resistant belt 3 approaches one side. It acts as a belt meandering prevention means that abuts against the wall 4a and regulates the deviation of the heat-resistant belt 3 to prevent meandering. The belt stretching member 4 is provided with a spring 9 between the heating roller 1 of the projecting wall 4a and the frame at the opposite end, and is lightly pressed by the spring 9 against the heating roller 1 and is in sliding contact with the heating roller 1. Positioned.

  Further, the belt stretching member 4 is inserted into the inner periphery of the heat-resistant belt 3 to apply tension to the heat-resistant belt 3 in cooperation with the pressure roller 2 and wind the heat-resistant belt 3 around the heating roller 1 to form a nip. A belt-shaped sliding member (a heat-resistant belt 3 slides on the belt tension member 4) disposed at a position to be formed. The belt sliding member 4 may be made of a metal such as an aluminum alloy having excellent thermal conductivity, but is preferably made of a plastic having excellent thermal conductivity from the viewpoint of shortening the warm-up time. As a more preferable condition, the surface of the plastic belt sliding member is subjected to a metal coating treatment such as copper plating, nickel plating or vapor deposition for the purpose of improving thermal conductivity, and the axial length is also set. If the length of the elastic layer covering portion of the heating roller 1 is shorter than the axial length, it is possible to further exhibit characteristics excellent in thermal response under a small heat capacity.

  FIG. 3 is a view for explaining another embodiment of the fixing device according to the present invention applied to a straight roller nip, and FIG. 4 is another embodiment of the fixing device according to the present invention applied to a crown-shaped roller nip. FIG. 5 shows another embodiment of the fixing device according to the present invention in which the length of the gap between the reverse-crown-shaped roller nip and the stretch member nip is substantially the same at the center and the end in the axial direction. FIG. 6 shows another embodiment of the fixing device according to the present invention, which is applied to a straight-shaped roller nip and has the same overall nip width and gap length at the central portion and the end portion in the axial direction. FIG. 7 shows another embodiment of the fixing device according to the present invention in which the length of the gap between the crown-shaped roller nip and the stretching member nip is substantially the same at the center and the end in the axial direction. It is a figure explaining a form.

  The above embodiment is a case where the roller nip formed between the heating roller 1 and the pressure roller 2 has an inverted crown shape, but also when the roller nip is a straight shape, as shown in FIG. In the case of the crown shape as well, as shown in FIG. 4, the tension member nip formed between the heating roller 1 and the belt tension member is similarly set so that the entire nip width is wider than the end portion in the axial center. Form. By forming the entire nip width including the roller nip and the stretch member nip so as to be wider at the center in the axial direction than at the end as in each of these embodiments, a toner image is formed in the same manner as in the above embodiments, and the heat capacity of the sheet The amount of heat that is insufficient at the central portion in the axial direction in which the ink becomes large can be compensated for, and gloss and fixing properties are uniform and stable printing can be obtained.

  The gap between the roller nip and the tension member nip is the part where the belt is in contact with the heating roller without any backup, and the pressing force is very small, so the orientation of the sheet material is likely to change, and the toner Heat transfer to is likely to be unstable. This unstable state cannot be resolved simply by forming the entire nip width composed of the roller nip and the stretching member nip so as to be wider at the center in the axial direction than at the end.

  In the embodiment shown in FIG. 5, the total nip width composed of the reverse crown-shaped roller nip and the stretching member nip is made wider at the axial center than the end, and the length of the gap between the roller nip and the stretching member nip 6 is made substantially the same at the axially central portion and the end portion, and in the embodiment shown in FIG. 6, the total nip width composed of the straight roller nip and the stretching member nip is made wider at the axially central portion than the end portion. At the same time, the length of the gap between the roller nip and the stretch member nip is made substantially the same at the center and the end in the axial direction. Furthermore, the width of the entire nip consisting of the roller nip and the tension member nip is made wider at the axial center than the end corresponding to the crown-shaped roller nip, and the length of the gap between the roller nip and the tension member nip is set in the axial direction. FIG. 7 shows an embodiment in which the central portion and the end portion are substantially the same. In this way, by making the total nip width and the gap length between the nips substantially the same at the central portion and the end portion in the axial direction, unevenness in heat transfer to the toner can be eliminated at the central portion and the end portion in the axial direction. In addition, the posture of the sheet material is stabilized, and gloss and fixing properties are uniform, and more stable printing can be obtained.

  FIG. 8 is a view for explaining the heat distribution of a heating roller having a built-in 1050 W and two columnar halogen lamps 1a as a heating source. An example of a heat distribution distribution in which one heating source built in the heating roller 1 is arranged as a central heater and the other one is used as an end heater, and the central portion is higher than the end portion is shown. FIG. 8 shows the heat distribution of the center heater and the edge heater with respect to the sheet passing area, and FIG. 8B shows the heat distribution of the combined heater. Further, the power consumption of the central heater is 600 W, the power consumption of the end heater is 450 W, and the heat distribution peak of the central heater is in a state where the heat distribution of the central heater and the end heater is synthesized. In contrast, the filaments are arranged so that the heat distribution peak of the end heater is 70 to 90%. In this way, by setting the heater heat distribution so that the center is high, the heat transfer amount to the toner can be supplemented at the central portion in the axial direction where heat is easily taken due to the heat capacity of the sheet material and the toner, Stable printing can be obtained with uniform gloss and fixing properties at the central and end portions in the axial direction.

  FIG. 9 is a schematic cross-sectional view of the overall configuration showing an embodiment of an image forming apparatus according to the present invention. In the figure, 10 is an image forming apparatus, 10a is a housing, 10b is a door, 11 is a paper transport unit, 15 is a cleaning means, 17 is an image carrier, 18 is an image transfer transport means, 20 is a developing means, and 21 is a scanner. Means, 30 is a paper feeding unit, 40 is fixing means, W is an exposure unit, and D is an image forming unit.

  In FIG. 9, the image forming apparatus 10 includes a housing 10a, a paper discharge tray 10c formed on the upper portion of the housing 10a, and a door body 10b attached to the front surface of the housing 10a so as to be freely opened and closed. 1 includes an exposure unit (exposure means) W, an image forming unit D, a transfer belt unit 29 having an image transfer and conveyance means 18, and a paper feed unit 30, and a paper conveyance unit 11 is arranged in the door body 10b. ing. Each unit has a configuration that can be attached to and detached from the main body, and can be removed and repaired or replaced integrally during maintenance or the like.

  The image forming unit D includes image forming stations Y (for yellow), M (for magenta), C (for cyan), and K (for black) that form a plurality (four in this embodiment) of different color images. I have. In each of the image forming stations Y, M, C, and K, an image carrier 17 that is a photosensitive drum, a charging unit 19 that is a corona charging unit and a developing unit that are disposed around the image carrier 17 are provided. Means 20 are included. These image forming stations Y, M, C, and K are arranged in parallel on the lower side of the transfer belt unit 29 so that the image carrier 17 faces upward along an oblique arched line. The order of arrangement of the image forming stations Y, M, C, and K is arbitrary.

  The transfer belt unit 29 is disposed below the housing 10a and is driven to rotate by a drive source (not shown), a driven roller 13 disposed obliquely above the drive roller 12, a tension roller 14, An image transfer conveying means 18 comprising an intermediate transfer belt that is stretched between these three and at least two rollers and is driven to circulate in the direction of the arrow in the figure, and a cleaning means 15 that contacts the surface of the image transfer conveying means 18. I have. The driven roller 13, the tension roller 14, and the image transfer / conveying means 18 are disposed in a direction inclined to the left in the drawing with respect to the driving roller 12, so that the belt conveying direction when driving the image transfer / conveying means 18 is downward. The belt surface 18a is positioned below, and the belt surface 18b whose transport direction is upward is positioned above.

  Accordingly, the image forming stations Y, M, C, and K are also arranged in a direction inclined to the left in the drawing with respect to the driving roller 12. Then, the image carrier 17 is brought into contact with the belt surface 18a facing downward in the transport direction of the image transfer transport unit 18 along the arched line, and is rotationally driven in the transport direction of the image transfer transport unit 18 as indicated by the arrows in the drawing. The The flexible endless sleeve-shaped image transfer / conveyance means 18 is brought into contact with the image carrier 17 from substantially the same wrapping angle so as to be covered from above, so that the image carrier 17 and the image transfer / conveyance means 18 are in contact with each other. The contact pressure and the nip width can be adjusted by controlling the tension applied to the image transfer / conveying means 18 by the tension roller 14, the arrangement interval of the image carrier 17, the winding angle (arch curvature), and the like. .

The drive roller 12 also serves as a backup roller for the secondary transfer roller 39. For example, a rubber layer having a thickness of about 3 mm and a volume resistivity of 10 ⁵ Ω · cm or less is formed on the peripheral surface of the drive roller 12, and the secondary transfer roller is grounded through a metal shaft. This is a conductive path for the secondary transfer bias supplied through the terminal 39. As described above, by providing the driving roller 12 with a rubber layer having high friction and shock absorption, it is difficult for the impact when the sheet material enters the secondary transfer portion to be transmitted to the image transfer conveying means 18, and the image quality is deteriorated. Can be prevented. Further, by making the diameter of the driving roller 12 smaller than the diameters of the driven roller 13 and the backup roller 14, the sheet material after the secondary transfer can be easily peeled off by the elastic force of the sheet material itself. Further, the driven roller 13 is also used as a backup roller for the cleaning means 15 described later.

  Note that the image transfer conveying means 18 is disposed in a direction inclined to the right in the drawing with respect to the driving roller 12, and correspondingly, each of the image forming stations Y, M, C, K is also illustrated with respect to the driving roller 12. And may be arranged along an oblique arch shape in a direction inclined to the right side.

  The cleaning unit 15 is provided on the side of the belt surface 18a facing downward in the transport direction, and a cleaning blade 15a that removes toner remaining on the surface of the image transfer transport unit 18 after the secondary transfer, and a toner transport that transports the collected toner A member 15b is provided. The cleaning blade 15 a is in contact with the image transfer / conveyance means 18 at a portion where the image transfer / conveyance means 18 is wound around the driven roller 13. Further, a primary transfer member 16 is brought into contact with the back surface of the image transfer conveying unit 18 so as to face an image carrier 17 of each of the image forming stations Y, M, C, and K described later. A transfer bias is applied.

  The exposure means W is disposed in a space formed obliquely below the image forming unit D disposed in the oblique direction. A paper feeding unit 30 is disposed below the exposure unit W and at the bottom of the housing 10a. The exposure means W is entirely housed in a case, and the case is disposed in a space formed obliquely below the belt surface facing downward in the transport direction. A single scanner means 21 comprising a polygon mirror motor 21a and a polygon mirror (rotating polygon mirror) 21b is horizontally disposed at the bottom of the case, and a plurality of laser light sources 23 modulated by image signals of respective colors. In the optical system B that reflects and scans the laser beam by the polygon mirror 21b and deflects and scans the image carrier, the single f-θ lens 22 and the scanning light path of each color are not parallel to the image carrier 17 respectively. A plurality of reflecting mirrors 24 are arranged so as to be folded.

  In the exposure means W configured as described above, an image signal corresponding to each color is emitted from the polygon mirror 21b with a laser beam modulated and formed based on a common data clock frequency, and the f-θ lens 22 and the reflection mirror 24 are passed through. After that, the image carrier 17 of each image forming station Y, M, C, K is irradiated to form a latent image. By providing the reflection mirror 24, the scanning optical path can be bent, the height of the case can be lowered, and the optical system can be made compact. In addition, the reflection mirror 24 is arranged so that the scanning optical path lengths to the image carrier 17 of the image forming stations Y, M, C, and K are the same. In this way, the length of the optical path from the polygon mirror 21b of the exposure means W to the image carrier 17 to the image carrier 17 (optical path length) for each image forming unit D is configured to be substantially the same, thereby scanning in each optical path. The scanning widths of the emitted light beams are also substantially the same, and no special configuration is required for image signal formation. Therefore, the laser light source can be modulated and formed based on a common data clock frequency even though it is modulated corresponding to images of different colors by different image signals, and uses a common reflecting surface. Color misregistration caused by a relative difference in the sub-scanning direction can be prevented, and a simple and inexpensive color image forming apparatus can be configured.

  Further, in this embodiment, by arranging the scanning optical system below the apparatus, the vibration of the scanning optical system due to the vibration that the drive system of the image forming means gives to the frame that supports the apparatus can be minimized. Degradation of image quality can be prevented. In particular, by arranging the scanner means 21 at the bottom of the case, the vibration that the polygon motor 21a itself gives to the entire case can be minimized, and deterioration of the image quality can be prevented. Further, by making the number of polygon motors 21a, which are vibration sources, one, vibration applied to the entire case can be minimized.

  The sheet feeding unit 30 includes a sheet feeding cassette 35 in which sheet materials are stacked and held, and a pickup roller 36 that feeds sheet materials one by one from the sheet feeding cassette 35. The paper transport unit 11 includes a gate roller pair 37 (one roller is provided on the housing 10a side) that regulates the timing of feeding the sheet material to the secondary transfer unit, the drive roller 12, and the image transfer transport means 18. A secondary transfer roller 39 as a secondary transfer unit that is pressed against the main body, a main recording medium conveyance path 38, a fixing unit 40, a discharge roller pair 41, and a duplex printing conveyance path 42 are provided.

  The secondary image (unfixed toner image) secondarily transferred to the sheet material is fixed at a predetermined temperature at the nip portion formed by the fixing unit 40. In this example, the fixing means 40 is disposed in a space formed obliquely above the belt surface 18b facing upward in the transfer belt conveyance direction, in other words, in a space opposite to the image forming station with respect to the transfer belt. Therefore, heat transfer to the exposure unit W, the image transfer conveyance unit 18, and the image forming unit can be reduced, and the frequency of performing the color misregistration correction operation for each color can be reduced. In particular, the exposure means W is located farthest from the fixing means 40, and the displacement of the scanning optical system components due to heat can be minimized, thereby preventing color misregistration. In the present embodiment, since the image transfer conveying means 18 is arranged in a direction inclined with respect to the driving roller 12, a large space is generated in the right space in the figure, and the fixing means 40 can be arranged in that space. In addition, downsizing can be realized, and heat generated by the fixing unit 40 is transmitted to the exposure unit W, the image transfer conveyance unit 18 and the image forming stations Y, M, C, and K located on the left side. Can be prevented. Further, since the exposure unit W can be arranged in the space on the lower left side of the image forming unit D, the vibration of the scanning optical system of the exposure unit W due to the vibration applied to the housing 10a by the drive system of the image forming means is minimized. The image quality can be suppressed and deterioration of the image quality can be prevented.

  Further, the corona charging means 19 is adopted as the charging means in accordance with the absence of the cleaning means. When the charging unit is a roller, a small amount of primary transfer residual toner existing on the image carrier 17 accumulates on the roller to cause a charging failure, but the corona charging unit 19 which is a non-contact charging unit. Can prevent toner from adhering and can prevent charging failure.

  Further, in this embodiment, the intermediate transfer belt is configured to contact the image carrier 17 as the image transfer conveyance unit 18, but the sheet material is adsorbed to the surface to be conveyed and moved, and a toner image is formed on the surface of the sheet material. A sheet material conveyance belt that forms and conveys an image by sequentially superimposing and transferring the image may be configured to contact the image carrier 17 as the image transfer conveyance means 18. In this case, the belt conveyance direction of the sheet material conveyance belt as the image transfer conveyance means 18 is upward in the opposite direction on the lower surface in contact with the image carrier 17.

  In addition, this invention is not limited to the said embodiment, A various deformation | transformation is possible. For example, in the above-described embodiment, the total nip width on the heating roller contact surface of the belt stretching member that forms the stretching member nip on both sides in the sheet material conveyance direction depends on the shape of the belt nip from the end in the axial center. Although it is formed to be large, the entire nip width is axially on either the upstream side (nip start portion) or the downstream side (nip end portion) in the sheet material conveyance direction on the heating roller contact surface of the belt stretching member You may form so that it may become larger than an edge part in a center part.

1 is a diagram illustrating an embodiment of a fixing device according to the present invention. It is a figure explaining the shape of the contact surface of a belt stretching member, and the shape of a stretching member nip and a roller nip. It is a figure explaining other embodiment of the fixing device based on this invention applied to a straight-shaped roller nip. It is a figure explaining other embodiment of the fixing device based on this invention applied to a crown-shaped roller nip. FIG. 10 is a diagram illustrating another embodiment of the fixing device according to the present invention in which the length of the gap between the reverse crown-shaped roller nip and the stretch member nip is substantially the same at the axial center and end. FIG. 10 is a diagram for explaining another embodiment of the fixing device according to the present invention, which is applied to a straight roller nip and has the same overall nip width and gap length at the center and end in the axial direction. FIG. 10 is a diagram for explaining another embodiment of the fixing device according to the present invention in which the length of the gap between the crown-shaped roller nip and the stretching member nip is substantially the same at the axial center and the end. It is a figure explaining the heat distribution of the heating roller which incorporated 1050 W as a heat source and the two columnar halogen lamps 1a. 1 is a schematic cross-sectional view showing an embodiment of an image forming apparatus according to the present invention. It is a figure explaining the example of formation of the main nip, the auxiliary nip, and the gap between nips.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Heating roller, 1a ... Halogen lamp, 1b ... Roller core metal, 1c ... Elastic layer, 2 ... Pressure roller, 2b ... Roller core metal, 2c ... Elastic layer, 3 ... Heat-resistant belt, 4 ... Belt tension member, 4a ... protruding wall, 4-N ... heating roller contact surface, 5 ... sheet material, 5a ... unfixed toner image, L ... pressing part tangent

Claims (10)

A heating roller, a pressure roller that presses against the heating roller, a heat-resistant belt that is clamped around the pressure roller and moves between the heating roller, and a belt tension that stretches the heat-resistant belt A fixing device that fixes an unfixed toner image formed on a sheet material, a nip formed between the heating roller and the pressure roller, the heating roller, and the belt stretching member. A fixing device characterized in that a total nip width formed of nips formed between the two is larger at an axially central portion than at an end portion. The fixing device according to claim 1, wherein the heating roller has a plurality of heating sources, and a heat distribution by the plurality of heating sources is made larger at an axially central portion than at an end portion. The fixing device according to claim 1, wherein the pressure roller has an inverted crown shape in which an outer diameter of an end portion is larger than a central portion. The pressure roller presses the heating roller with a predetermined pressing force, and the belt stretching member forms a nip by lightly pressing the heating roller with a pressing force smaller than the pressure roller. The fixing device according to claim 1. The fixing device according to claim 4, wherein the belt stretching member is configured to lightly press the heating roller by a frictional force with the heat resistant belt when the heat resistant belt is rotationally driven. The belt stretching member is configured to set a rotation fulcrum so as to be swingable in the rotation center direction of the heating roller, and to apply a tension in a tangential direction to a nip portion of the heating roller to tension the heating belt. The fixing device according to any one of claims 1 to 5, wherein the fixing device is configured to be laid and swing when paper is passed to press the heating roller. The fixing device according to claim 1, wherein the belt stretching member is disposed on the upstream side in the sheet material conveying direction with respect to the pressure roller. The belt stretching member forms a nip by winding the heat-resistant belt around the heating roller with the pressure roller, and contacts the heating roller along the nip formed with the heating roller. 8. The fixing device according to claim 1, wherein the fixing device is formed so that a central portion has a high convex shape with respect to both end portions in the axial direction of the contact surface. The fixing device according to claim 1, wherein the belt stretching member is a substantially meniscus belt sliding member. An image forming apparatus comprising the fixing device according to claim 1.

Images