Scanning optical apparatus and image forming apparatus

Abstract

The objective is to improve quietness by achieving both the disruption of vortices generated by the rotation of the rotating polyhedron mirror and the suppression of wind blowing onto the edges of the rotating polyhedron mirror, all within an inexpensive configuration. [Solution] The cover member 20 has a plurality of ribs 21-1 to 21-4 that protrude from the ceiling wall surface 20a toward the rotating polyhedron mirror 4. The plurality of ribs 21-1 to 21-4 are arranged to have rotational symmetry with respect to the rotation axis of the rotating polyhedron mirror 4 when viewed in the direction of the rotation axis. They are positioned at a predetermined distance L5 upstream of the rotating polyhedron mirror 4 in the direction of rotation with respect to a plurality of imaginary lines L1 to L4 that extend from the rotation axis toward the circumscribed circle Cc of the rotating polyhedron mirror 4 and have the same number of rotational symmetries as the number of ribs 21-1 to 21-4. The ribs 21-1 (~21-4) are elongated in shape parallel to the imaginary line L1 (~L4), with surface 21a positioned outside the circumscribed circle Cc and surface 21b positioned inside the circumscribed circle Cc.

Description

Technical Field

[0001] The present invention relates to a scanning optical device and an image forming apparatus, and more particularly to a scanning optical device that performs optical writing using a laser beam in an image forming apparatus such as a laser beam printer, a digital copier, or a digital facsimile.

Background Art

[0002] A scanning optical device is used in an electrophotographic image forming apparatus. The scanning optical device deflects and scans a light beam emitted from a semiconductor laser by a rotating polygon mirror, and guides the deflected and scanned light beam onto a photoreceptor by optical components such as lenses and mirrors. Thereby, an electrostatic latent image is formed on the photoreceptor. The rotating polygon mirror rotates when deflecting and scanning the light beam inside a housing (hereinafter referred to as an optical box). In recent years, as the speed of image forming apparatuses has increased, the demand for high-speed rotation of the rotating polygon mirror in the scanning optical device has been increasing. When the rotating polygon mirror is rotated at high speed, there is a problem that the operating noise (hereinafter referred to as wind noise and rotation noise) caused by the rotating polygon mirror increases and the quietness is impaired. One of the causes of the rotation noise is an air flow (hereinafter referred to as a vortex) that circulates annularly around the rotation axis of the rotating polygon mirror generated as the rotating polygon mirror rotates. The vortex is generated when the air pushed out by the rotation of the rotating polygon mirror is drawn in while rising toward the vicinity of the rotation center where the negative pressure is generated due to the rotation. As a countermeasure against vortex generation, a configuration has been proposed in which ribs protrude from the cover member of the scanning optical device to the vicinity of the rotating polygon mirror to block (divide) the vortex generated as the rotating polygon mirror rotates, thereby improving the quietness. The configuration of the ribs includes a configuration in a cross shape (for example, see Patent Document 1) and a configuration in which a plurality of independent ribs are arranged radially from the center of the rotation axis of the rotating polygon mirror (for example, see Patent Document 2).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

[0004] However, in conventional designs, the fluid fluctuations (pressure fluctuations) that cause wind noise (rotational noise) when the rotating polyhedron mirror rotates are large, which presents a problem of increased rotational noise. The fluid fluctuations (pressure fluctuations) become large for the following reasons: When the rotating polyhedron mirror rotates, the air pushed out by the corners formed by adjacent reflective surfaces of the rotating polyhedron mirror is drawn upward towards the rotation axis center of the rotating polyhedron mirror, where the pressure becomes negative as it rotates. Subsequently, the rising air hits the cover member and descends again, blowing towards the vicinity of the corner (edge) formed by the top surface and reflective surface of the rotating polyhedron mirror. This strengthens the downward flow (shear flow) in the direction of the rotation axis of the rotating polyhedron mirror.

[0005] This invention was made under these circumstances and aims to improve quietness by achieving both the disruption of vortices generated by the rotation of a rotating polyhedron mirror and the suppression of wind blowing onto the edges of the rotating polyhedron mirror, all with an inexpensive configuration. [Means for solving the problem]

[0006] To solve the above-mentioned problems, the present invention has the following configuration.

[0007] (1) A scanning optical apparatus comprising: a light source that emits a light beam; a rotating polyhedron mirror that deflects and scans the light beam emitted from the light source; a scanning lens that images the light beam deflected and scanned by the rotating polyhedron mirror onto a surface to be scanned; an optical box having an aperture that houses the light source, the rotating polyhedron mirror and the scanning lens; and a cover that covers the aperture, wherein the cover has a plurality of ribs formed on the surface of the cover that faces the rotating polyhedron mirror when it is covering the aperture, the ribs protruding from the surface toward the rotating polyhedron mirror, and the plurality of ribs are centered on the axis of rotation of the rotating polyhedron mirror when viewed in the direction of the rotation axis of the rotating polyhedron mirror. A scanning optical device characterized in that it is arranged to have rotational symmetry, extends from the axis of rotation toward the circumscribed circle of the rotating polyhedron, is the same number as the number of ribs, and is positioned at a predetermined distance upstream or downstream in the rotational direction of the rotating polyhedron with respect to the plurality of imaginary lines having rotational symmetry with respect to the axis of rotation, each of the ribs is elongated in shape parallel to the imaginary line corresponding to each of the ribs when viewed in the direction of the axis of rotation, one end of the rib in the longitudinal direction is positioned outside the circumscribed circle, and the other end in the longitudinal direction is positioned inside the circumscribed circle.

[0008] (2) An image forming apparatus for forming an image on a recording material, comprising a photoreceptor having the scanning surface and the scanning optical apparatus described in (1). [Effects of the Invention]

[0009] According to the present invention, it is possible to improve quietness by simultaneously disrupting the vortex flow generated by the rotation of the rotating polyhedron mirror and suppressing the wind blowing onto the edge of the rotating polyhedron mirror, all with an inexpensive configuration. [Brief explanation of the drawing]

[0010] [Figure 1] Diagram showing the configuration of the scanning optical apparatus in Examples 1 and 2. [Figure 2] A diagram showing the cover member of the scanning optical device in Example 1. [Figure 3]Diagram showing the assembly of the cover member of the scanning optical device in Examples 1 and 2. [Figure 4] Diagram showing the positional relationship between the rectifier plate and the rotating polyhedron mirror in Example 1. [Figure 5] This diagram shows the airflow generated when the rotating polyhedron mirror of Example 1 rotates. [Figure 6] Example 1: Diagram showing the airflow generated near the rectifier plate in a conventional example. [Figure 7] Example 1: A diagram showing the airflow generated when a conventional rotating polyhedron mirror rotates. [Figure 8] Example 1: Graph showing the results of a conventional noise measurement test. [Figure 9] Diagram showing the positional relationship between the rectifier plate and the rotating polyhedron mirror in Example 2, and diagram showing the airflow generated near the rectifier plate. [Figure 10] Diagram showing the configuration of the image forming apparatus in Example 3. [Modes for carrying out the invention]

[0011] Embodiments of the scanning optical device according to the present invention will be described in detail. [Examples]

[0012] [Scanning Optical Device] Figure 1 is a perspective view showing the configuration of the scanning optical device of Example 1. The scanning optical device 101 shown in Figure 1 is used in image forming devices such as laser beam printers, digital copiers, and facsimile machines to form an electrostatic latent image on the surface (scanned surface) of a photosensitive drum 8, which serves as an image carrier, using laser light (light beam). The scanning optical device 101 has a semiconductor laser unit 1 as a light source that emits a laser beam L, and an anamorphic collimator lens 2 which is integrally molded with a collimator lens and a cylindrical lens.

[0013] The scanning optical device 101 has a main scanning aperture 3 having a through groove and a rotating polygon mirror 4 having a shape of a regular quadrangular prism. The rotating polygon mirror 4 has a reflecting surface 11 for reflecting the laser beam L. In the first embodiment, the rotating polygon mirror 4 has four reflecting surfaces 11, but the number of the reflecting surfaces 11 is not limited to this. The rotating polygon mirror 4 has a top surface 43 of the rotating polygon mirror 4 and an edge portion 41 which is a corner formed by the top surface 43 and the reflecting surface 11. The rotating polygon mirror 4 has a corner portion 42 formed by the reflecting surfaces 11.

[0014] The scanning optical device 101 has an optical deflector 5 that rotationally drives the rotating polygon mirror 4 by a motor serving as a drive source. The scanning optical device 101 has a beam detector (hereinafter referred to as BD) 6 as a detection means for the laser beam L deflected and scanned by the optical deflector 5 in order to determine a writing start position of the laser beam L on the surface of the photosensitive drum 8. The scanning optical device 101 has a scanning lens 7 as an imaging means for forming an image of the deflected and scanned laser beam L on the surface (scanned surface) of the photosensitive drum 8, and a folding mirror 10 for deflecting the laser beam L that has passed through the scanning lens 7 toward the photosensitive drum 8.

[0015] The scanning optical device 101 has an optical box 9 and a cover member 20 (cover) (see FIG. 2). The optical box 9 is a housing that houses each of the above-described optical members including the optical deflector 5, and has an opening through which each of the above-described optical members passes when installed on the bottom surface or side surface of the optical box 9. The optical box 9 is made of a black resin and is formed by injection molding. The optical box ⑨ is provided with a positioning portion 91 for determining a relative position with respect to the cover member 20 that closes the opening of the optical box 9. The optical box 9 is provided with an emission port 12 for emitting the laser light to the outside.

[0016] The direction in which the laser beam L is scanned by the light deflector 5 is defined as the main scanning direction, and the direction orthogonal to the main scanning direction is defined as the sub-scanning direction. The operation of the scanning optical device 101 will be described below. The laser beam L emitted from the semiconductor laser unit 1 is made into substantially parallel light or converging light in the main scanning direction by the anamorphic collimator lens 2, and into converging light in the sub-scanning direction. Next, the beam width of the laser beam L in the main scanning direction is restricted by the main scanning aperture 3. Note that the beam width in the sub-scanning direction is restricted by an aperture (sub-scanning aperture, not shown) located upstream of the anamorphic collimator lens 2. The light beam that has passed through the sub-scanning aperture, the anamorphic collimator lens 2, and the main scanning aperture 3 forms an image in a focal line shape that extends long in the main scanning direction on the reflecting surface 11 of the rotating polygon mirror 4.

[0017] The laser beam L that has formed an image on the reflecting surface 11 of the rotating polygon mirror 4 is deflected and scanned by the rotating polygon mirror 4 that rotates in the direction of arrow A. The deflected and scanned laser beam L scans on the BD6 in the main scanning direction. Next, the deflected and scanned laser beam L enters the scanning lens 7. The laser beam L that has formed an image in a focal line shape on the rotating polygon mirror 4 becomes a spot of several millimeters on the scanning lens 7 and passes through the scanning lens 7. The laser beam L that has passed through the scanning lens 7 is deflected by the folding mirror 10 and is emitted outside the scanning optical device 101 from the emission port 12.

[0018] The laser beam L emitted outside the scanning optical device 101 finally scans in the direction of arrow B in FIG. 1 while forming a spot on the photosensitive drum 8. Here, sub-scanning is performed by rotationally driving the photosensitive drum 8 around the axis of its cylinder. Thereby, an electrostatic latent image corresponding to the image information is formed on the surface of the photosensitive drum 8 uniformly charged by a charging means (not shown).

[0019] [Cover member] Figure 2 is an explanatory diagram of a cover member 20 that is assembled to the optical box 9 and closes the opening of the optical box 9 that houses each optical component, in the scanning optical device 101 shown in Figure 1. In the cover member 20, the surface facing the rotating polyhedron 4 when covering the opening of the optical box 9 is called the ceiling wall surface 20a. The ceiling wall surface 20a can also be said to face the opening of the optical box 9. Rectifying plates 21 (ribs 21-1 to 21-4) are formed on the ceiling wall surface 20a. In addition, the cover member 20 is provided with a positioning part 201 for accurate positioning relative to the optical box 9.

[0020] Figure 3 is a perspective view showing the assembly of the cover member 20 into the optical box 9. As mentioned above, the optical box 9 is provided with a positioning section 91 and the cover member 20 is provided with a positioning section 201, allowing the components to be precisely positioned relative to each other when closing the opening of the optical box 9 with the cover member 20. This allows for the relative positioning of the rectifier plate 21 provided on the cover member 20 and the rotating polyhedron 4 housed in the optical box 9, thereby stably achieving the noise reduction effect of the rectifier plate 21 on the rotation of the rotating polyhedron 4.

[0021] Furthermore, the configuration in which the opening of the optical box 9 is closed by the cover member 20 provides the following effects. For example, by blocking both the leakage of the laser beam L from inside the scanning optical device 101 and the intrusion of light from the outside, safe and stable deflection scanning of the laser beam L can be achieved. In addition, by sealing the opening of the optical box 9 with the cover member 20 and preventing the rotational noise generated by the rotation of the rotating polyhedron mirror 4 from leaking to the outside of the scanning optical device 101, it is also possible to suppress the operating noise when the image forming apparatus is operating. In addition, by limiting the amount of air entering and leaving the scanning optical device 101, the possibility of dust floating in the air adhering to optical elements such as the rotating polyhedron mirror 4 can be reduced. This prevents dust from adhering to optical elements, which would locally reduce the amount of light deflected and scanned against the photosensitive drum 8, and thus prevent density unevenness from occurring in the printed material.

[0022] [Rectifier plate] Figure 4 will be used to explain the detailed shape of the rectifier plate 21 and its positional relationship with the rotating polyhedron mirror 4. Figure 4 shows the optical box 9 with the cover member 20 attached. The rotation axis direction of the rotating polyhedron mirror 4 is defined as the z direction, and the directions perpendicular to the z direction are defined as the x and y directions. In Example 1, the rotation axis direction of the photosensitive drum 8 (arrow B in Figure 1) is defined as the y direction.

[0023] Figure 4(a) shows the positional relationship between the rectifier plate 21 and the rotating polyhedron 4 when viewed from the side (+x direction). The rectifier plate 21 is positioned above the rotating polyhedron 4 (+z direction). More specifically, the rectifier plate 21 is composed of a plurality of ribs 21-1 to 21-4 that protrude from the ceiling wall surface 20a of the cover member 20 toward the rotating polyhedron 4, and the plurality of ribs 21-1 to 21-4 protrude to a length that does not touch the top surface 43 of the rotating polyhedron 4. In other words, the rectifier plate 21 is provided between the ceiling wall surface 20a of the cover member 20 and the top surface 43 of the rotating polyhedron 4.

[0024] Figure 4(b) is a cross-sectional view of Figure 4(a), showing the positional relationship between the rectifier plate 21 and the rotating polyhedron 4 when viewed with respect to the -z direction. The rotating polyhedron 4 rotates counterclockwise (direction A) when deflecting and scanning the laser beam L (not shown). The rectifier plate 21 consists of a plurality of independent linear ribs 21-1 to 21-4, and each rib 21-1 to 21-4 is arranged point-symmetrically with respect to the rotation axis center O of the rotating polyhedron 4. In Embodiment 1, the rectifier plate 21 has four ribs 21-1 to 21-4.

[0025] In Example 1, the rectifier plate 21 is offset (translated) upstream with respect to the rotation direction (direction A) of the rotating polyhedron 4. Here, as shown in Figure 4(b), when viewed in the direction of the rotation axis, let L1, L2, L3, and L4 be four imaginary lines that pass through the rotation axis center O of the rotating polyhedron 4, have rotational symmetry, and extend toward the circumscribed circle Cc of the rotating polyhedron 4 (see Figure 4). In this case, the four ribs 21-1 to 21-4 are not on the imaginary lines L1 to L4, but are located upstream of the imaginary lines L1 to L4 in the direction of rotation, at a predetermined distance L5 upstream in the direction of rotation, in other words, at offset positions.

[0026] More specifically, rib 21-1 is not on the virtual line L1, but is located at a predetermined distance L5 upstream of the virtual line L1 in the direction of rotation. Rib 21-2 is not on the virtual line L2, but is located at a predetermined distance L5 upstream of the virtual line L2 in the direction of rotation. Rib 21-3 is not on the virtual line L3, but is located at a predetermined distance L5 upstream of the virtual line L3 in the direction of rotation. Rib 21-4 is not on the virtual line L4, but is located at a predetermined distance L5 upstream of the virtual line L4 in the direction of rotation. Since the virtual lines L1 and L2, L2 and L3, L3 and L4, and L4 and L1 are all orthogonal to each other, there is a 90° angle in the direction of rotation between a rib and an adjacent rib in the direction of rotation of the rotating polyhedron 4. That is, the four ribs 21-1 to 21-4 are arranged radially around the rotation axis center O at 90° intervals.

[0027] Furthermore, the surface 21a (one end) of the rectifier plate 21 on the side furthest from the rotation axis center O is inclined. For example, in rib 21-1, surface 21a slopes downward to the right. That is, of the two surfaces 21c and 21d parallel to the imaginary line L1, surface 21a is inclined to approach the rotation axis center O from surface 21c, which is closer to the imaginary line L1, to surface 21d, which is farther away from the imaginary line L1. The other ribs 21-2 to 21-4 have a similar inclination with respect to the imaginary lines L2 to L4 and the rotation axis center O. By inclining surface 21a, care is taken to ensure that the air that collides with the end surface 21a is smoothly rectified.

[0028] Furthermore, the inclination of surface 21a may be in the opposite direction (downward to the left), and it is sufficient that it is not configured to be perpendicular to (not inclined to) the imaginary lines L1 to L4, as in the conventional example described later. Moreover, the inclination of surface 21a is not limited to a straight inclination when viewed in the direction of the rotation axis, as shown in Figure 4(b). Surface 21a may be inclined in a curved shape, for example, or in other words, surface 21a may be a curved surface. Also, in Embodiment 1, the rectifier plate 21 is configured such that ribs 21-1 and 21-3 are parallel to the x direction and 21-2 and 21-4 are parallel to the y direction, but it is not limited to this. The relationship between ribs 21-1 to 21-4 and the x and y directions does not have to be parallel. That is, in Figure 2, the rectifier plate 21 is sufficient if it is provided radially around the rotation axis center O of the rotating polyhedron 4 and with a predetermined offset, and it may also be provided rotated from the state shown in Figure 2 around the rotation axis center O in the range of 0° to 90°.

[0029] Here, let Cc be the circumscribed circle of the rotating polyhedron 4. As shown in Figure 4(b), the ribs 21-1 to 21-4 are provided such that the surface 21a of the ribs 21-1 to 21-4 is located outside the circumscribed circle Cc, and the surface 21b (the other end) is located inside the circumscribed circle Cc. This is because if the surface 21a is inside the circumscribed circle Cc, it will not be possible to rectify the airflow toward the outside of the corner 42, and therefore the upward airflow will not be rectified. Also, if the surface 21b is outside the circumscribed circle Cc, it will not be possible to rectify the airflow toward the rotation axis center O. In both cases, there is a risk of reduced noise levels. Furthermore, the amount of the offset (distance L5) described above may be determined within a range that satisfies the conditions that surface 21a is outside the circumscribed circle Cc and surface 21b is inside the circumscribed circle Cc.

[0030] Furthermore, the surface 21b of the end of the rectifier plate 21 closest to the rotation axis center O has a curved shape that forms part of a virtual circle Sc centered on the rotation axis center O. By forming part of the virtual circle Sc concentrically from the rotation axis center O, the surface 21b can smoothly rectify the swirling air, preventing turbulence from occurring near the rotation axis center O. For example, if the surface 21b of rib 21-1 is perpendicular to the virtual line L1 (in other words, the rib is rectangular), vortices may collide with the 90° corner of the surface 21b offset from the virtual line L1, potentially making it impossible to control the airflow. In Embodiment 1, turbulence is prevented by making the surface 21b a curved surface that forms part of the virtual circle Sc.

[0031] In Example 1, the number of ribs is set to four and is arranged at 90° intervals around the rotation axis center O, but this is not limited to this. The number of ribs may be five or more. For example, the number of ribs may be the same as the number of faces of the rotating polyhedron 4. In Example 1, the number of ribs on the rectifier plate 21 is set to four, the same as the number of faces of the rotating polyhedron 4. For example, if the number of faces of the rotating polyhedron 4 is five, five ribs may be provided at positions offset by a predetermined distance upstream in the direction of rotation (arrow A) from five imaginary lines drawn radially around the rotation axis center O at 72° intervals, passing through the rotation axis center O.

[0032] In this embodiment 1, the multiple ribs are arranged to have rotational symmetry with respect to the rotation axis center O. Furthermore, the multiple ribs are positioned at a predetermined distance upstream in the rotational direction of the rotating polyhedron 4 with respect to a number of imaginary lines that extend from the rotation axis center O toward the circumscribed circle Cc of the rotating polyhedron 4 and have rotational symmetry equal to the number of ribs. Alternatively, the multiple ribs may be positioned at a predetermined distance downstream in the rotational direction of the rotating polyhedron 4 with respect to a number of imaginary lines that extend from the rotation axis center O toward the circumscribed circle Cc of the rotating polyhedron 4 and have rotational symmetry equal to the number of ribs, as will be explained in embodiment 2. In terms of rotational symmetry, the ribs 21-1 to 21-4 in embodiment 1 are 4-fold symmetry, while the rectifier plate with the five ribs described above is 5-fold symmetry.

[0033] Each rib has an elongated shape parallel to the imaginary line corresponding to that rib, with one end (21a) in the longitudinal direction positioned outside the circumscribed circle Cc and the other end (21b) in the longitudinal direction positioned inside the circumscribed circle Cc. One end (21a) is inclined from upstream to downstream in the direction of rotation when viewed in the direction of the rotation axis. This inclination may be formed by a curved surface. The other end (21b) is formed by a curved surface such that, when viewed in the direction of the rotation axis, it forms part of the circumference of the imaginary circle Sc centered on the rotation axis of the rotating polyhedron 4. The rotating polyhedron 4 has a plurality of reflective surfaces 11 that reflect the light beam, and the number of ribs may be equal to the number of reflective surfaces 11. When the rotating polyhedron 4 has an even number of reflective surfaces that reflect the light beam, the ribs are arranged point-symmetrically with respect to the rotation axis.

[0034] [Airflow] Here, the inventors of the present invention visualized the airflow when the rotating polyhedron 4 rotates using a fluid analysis model. The airflows illustrated in the following description are all schematic representations of the analysis results. The airflow when the rotating polyhedron 4 rotates will be explained in detail using Figure 5. Figure 5(a) is a perspective view showing the rotating polyhedron 4 rotating counterclockwise (direction A) around the rotation axis center O. Figure 5(b) shows the vortex flow Wrot that occurs near the top surface 43 of the rotating polyhedron 4 when it rotates. When the rotating polyhedron 4 rotates, the flow due to vortex nuclei and vortex filaments with equal rotational angular velocity has the fastest flow velocity near the rotation axis center O, so the pressure is small, resulting in a negative pressure region Ra shown by the dashed line (pump effect).

[0035] Figure 5(c) shows the rising vortex flow Wup of air generated when the rotating polyhedron 4 rotates. The corners 42 formed by adjacent reflective surfaces 11 of the rotating polyhedron 4 strongly push out air Wa mainly in the tangential direction of the circumscribed circle Cc of the rotating polyhedron 4 near the edge 41. Due to the influence of the negative pressure region Ra near the rotation axis center O, air Wa is drawn into a vortex towards the rotation axis center O and rises rapidly near the rotating polyhedron 4.

[0036] Figure 5(d) shows the downward airflow Wdown generated when the rotating polyhedron 4 rotates. As shown in Figure 5(c), the air (Wup) that has risen once moves along the rectifier plate 21 and the ceiling wall surface 20a of the cover member 20 (neither shown) toward the center of the rotating polyhedron 4, and near the center it begins to descend, blowing down from above the rotating polyhedron 4. The descending air flows again toward the circumscribed circle Cc of the rotating polyhedron 4 along the rotation axis center O and the top surface 43 of the rotating polyhedron 4. However, some of the air descends near the edge portion 41 and collides with the air Wup that is being pushed up by the corner portion 42 formed by the reflective surfaces 11 of the rotating polyhedron 4, causing turbulence.

[0037] [Airflow and rotational noise] Next, we will explain the relationship between the aforementioned airflow and the operating noise (rotational noise) generated when the rotating polyhedron 4 rotates. One of the causes of the rotational noise is the swirling air Wrot generated near the top surface 43 of the rotating polyhedron 4, and the air Wup that is pushed out in a swirling motion by the corner 42 formed by the reflective surfaces 11 of the rotating polyhedron 4 near the edge 41. By dividing this swirling air Wrot and Wup with the rectifier plate 21 and reducing the size of the vortex, the generation of rotational noise can be suppressed.

[0038] Another cause of the rotational noise is turbulence, which occurs when air Wdown descending near the edge 41 collides with swirling air Wup being pushed upward by the corner 42 formed by the reflective surfaces 11 of the rotating polyhedron 4. The fact that turbulence is the cause of the rotational noise can be inferred from the fact that fluid fluctuations (pressure fluctuations) become large when the shear flow component with respect to the rotation axis of the rotating polyhedron 4 is large. To reduce this turbulence, the following is effective: In other words, it is effective to suppress the flow velocity of air Wdown descending toward the edge 41 in order to reduce the collision energy between the airs, and to straighten the swirling air Wup toward the rotation axis center O with the straightening plate 21 before it starts to descend.

[0039] [Airflow and rectifiers] The relationship between the airflow when the rotating polyhedron 4 rotates and the rectifier plate 21 will be explained using Figure 6. Figure 6 is a cross-sectional view of the rectifier plate 21 when the rotating polyhedron 4 is viewed in the -z direction. Figure 6(a) shows the airflow near the rectifier plate 121 of a conventional example when the rotating polyhedron 4 (not shown) rotates counterclockwise (direction A). Figure 6(b) shows the airflow near the rectifier plate 21 of Example 1 when the rotating polyhedron 4 (not shown) rotates counterclockwise (direction A).

[0040] In this example, the conventional rectifier plate 121 has four independent ribs 121-1 to 121-4. The conventional rectifier plate 121 is not offset upstream with respect to the rotation direction (direction A) of the rotating polyhedron 4. That is, the four ribs 121-1 to 121-4 are each on imaginary lines L1 to L4 passing through the rotation axis center O of the rotating polyhedron 4. Also, for example, both ends of rib 121-1 in the direction along imaginary line L1 (longitudinal direction) are perpendicular to imaginary line L1, and the shape of rib 121-1 is rectangular. The same applies to the other ribs 121-2 to 121-4.

[0041] As the rotating polyhedron 4 (not shown) rotates, air is drawn in a vortex towards the rotation axis center O, as described above. During the process of being drawn in a vortex, the air flowing along the rectifier plates 21 and 121 (dashed arrows in Figures 6(a) and 6(b)) is straightened by the rectifier plates 21 and 121. As a result, the air velocity decreases, and the air flows along the rectifier plates 21 and 121 towards the rotation axis center O. On the other hand, the air that is drawn in directly towards the rotation axis center O without flowing along the rectifier plates 21 and 121 (solid arrows in Figures 6(a) and 6(b)) is not straightened, and its velocity remains undiminished.

[0042] As mentioned above, the more rectified air there is, the smaller the shear flow component relative to the rotation axis of the rotating polyhedron 4 can be, resulting in a higher noise reduction effect. Comparing the conventional example with Example 1, it can be seen that more air flows along the rectifier plates 21 and 121 in Example 1 (More arrows shown by dashed lines in Example 1), indicating that the air is efficiently rectified toward the rotation axis center O. Conversely, more air flows directly toward the rotation axis center O without flowing along the rectifier plates 21 and 121 in the conventional example (More arrows shown by solid lines in the conventional example).

[0043] [Airflow velocity] Figure 7 illustrates the flow velocity of the wind (air) blown onto the edge portion 41 of the rotating polyhedron 4. As mentioned above, the inventors of this invention have visualized the airflow when the rotating polyhedron 4 rotates using a fluid analysis model.

[0044] Figure 7(a) is a schematic diagram showing the simulation results of the airflow velocity blown in the direction of the rotation axis against the edge portion 41 of the rotating polyhedron 4 after being straightened by the conventional example's straightening plate 121, and Figure 7(b) is a schematic diagram showing the airflow velocity blown in the direction of the rotation axis against the edge portion 41 of the rotating polyhedron 4 after being straightened by the straightening plate 21 of Example 1. The thickness of the arrows indicates the airflow velocity. The thicker the arrow, the greater the airflow velocity, and the thinner the arrow, the smaller the airflow velocity. From these results, it can be seen that in Example 1, the airflow velocity in the direction of the rotation axis against the edge portion 41 of the rotating polyhedron 4 is smaller than in the conventional example. This is because, as mentioned above, a high straightening effect was obtained by the arrangement of the straightening plate 21.

[0045] [Noise measurement results] Figure 8 is a graph showing the results of noise measurement tests conducted using the scanning optical device 101 of the conventional example and Example 1. In the test, sound-collecting microphones were placed around the scanning optical device 101, and the acoustic energy [%] of the sound measured by the sound-collecting microphones was graphed. According to these results, when the acoustic energy of the sound measured in the conventional example is set to 100%, it was approximately 60% in Example 1. In other words, it can be seen that the acoustic energy of the scanning optical device 101 of Example 1 has been reduced to about 60% compared to the conventional example. As mentioned above, this is because the rectification effect was greatly increased by the arrangement of the rectifier plate 21, which suppressed the generation of turbulence.

[0046] As described above, a flow straightening plate 21 having multiple independent linear ribs 21-1 to 21-4 is placed near the rotating polyhedron 4. This makes it possible to both divide vortices and suppress turbulence. Specifically, it is possible to divide the annular circulating vortex that is generated when air is drawn in while rising relative to the rotation axis of the rotating polyhedron 4. Furthermore, it is possible to suppress turbulence caused by the collision of air descending near the edge portion 41 of the rotating polyhedron 4 with vortex-shaped air being pushed upward by the corner portion 42 formed by the reflective surfaces 11 of the rotating polyhedron 4. This makes it possible to realize an optical scanning device with improved quietness.

[0047] As described above, according to Example 1, it is possible to improve quietness by achieving both the division of vortices generated by the rotation of the rotating polyhedron mirror and the suppression of wind blowing onto the edge of the rotating polyhedron mirror, using an inexpensive configuration. [Examples]

[0048] In Example 2, the configuration of the rectifier plate 31 and the airflow when the rotating polyhedron mirror 4 rotates will be described. Since the other device configurations and arrangement of components are the same, the explanation of individual functions will be omitted, and only the different configurations will be described. In addition, the same reference numerals will be used in the text to describe components that are the same as in Example 1.

[0049] [Rectifier plate] Figure 9(a) is a cross-sectional view showing the positional relationship between the rectifier plate 31 and the rotating polyhedron 4 when viewed with respect to the -z direction. The rotating polyhedron 4 rotates counterclockwise (direction A) when deflecting and scanning the laser beam L (not shown). The rectifier plate 31 has a plurality of independent linear ribs and is arranged point-symmetrically with respect to the rotation axis center O of the rotating polyhedron 4. In Embodiment 2, the rectifier plate 31 has four ribs 31-1 to 31-4.

[0050] In Embodiment 2, the rectifier plate 31 is offset downstream with respect to the rotation direction of the rotating polyhedron 4. That is, passing through the rotation axis center O of the rotating polyhedron 4, the four ribs 31-1 to 31-4 are not on the virtual lines L1 to L4, but are located at a distance L6 downstream of the virtual lines L1 to L4 in the direction of rotation.

[0051] More specifically, rib 31-1 is not on the virtual line L1, but is located at a predetermined distance L6 downstream from the virtual line L1 in the direction of rotation. Rib 31-2 is not on the virtual line L2, but is located at a predetermined distance L6 downstream from the virtual line L2 in the direction of rotation. Rib 31-3 is not on the virtual line L3, but is located at a predetermined distance L6 downstream from the virtual line L3 in the direction of rotation. Rib 31-4 is not on the virtual line L4, but is located at a predetermined distance L6 downstream from the virtual line L4 in the direction of rotation.

[0052] Furthermore, the surface 31a at the end of the rectifier plate 31 furthest from the rotation axis center O is inclined in the same way as surface 21a in Example 1, so that the air that collides with the end surface 31a can be smoothly rectified. Also, the surface 31b at the end of the rectifier plate 31 closer to the rotation axis center O is curved in the same way as in Example 1, forming a part of a virtual circle Sc concentric with the rotation axis center O of the rectifier plate 31, thereby preventing turbulence from occurring near the rotation axis center O.

[0053] [Airflow and rectifiers] Furthermore, similar to Example 1, the inventors of the present invention used a fluid analysis model to visualize the airflow in Example 2 when the rotating polyhedron 4 rotates. Figure 9(b) shows the airflow generated near the rectifier plate 31 in Example 2 when the rotating polyhedron 4 rotates, and is a cross-sectional view of the rectifier plate 31 when the rotating polyhedron 4 is viewed in the -z direction. When the rotating polyhedron 4 (not shown) rotates counterclockwise (direction A), as described above, the air is drawn in a vortex towards the rotation axis center O.

[0054] During the process of being drawn in in a vortex, the air flowing along the rectifier plate 31 (dashed arrow in Figure 9(b)) is rectified by the rectifier plate 31, reducing its velocity, and flows along the rectifier plate 31 toward the rotation axis center O. On the other hand, the air that is drawn in directly toward the rotation axis center O without flowing along the rectifier plate 31 (solid arrow in Figure 9(b)) is not rectified, and its velocity remains undiminished.

[0055] As mentioned above, the more rectified air there is, the smaller the shear flow component with respect to the rotation of the rotating polyhedron 4 can be, and thus a higher noise reduction effect can be obtained. Comparing the airflow of Example 2 shown in Figure 9(b) with the airflow of the conventional example shown in Figure 6(a), it can be seen that more air collides with the rectifier plate 31 in Example 2, and that Example 2 is able to efficiently rectify the airflow towards the rotation axis center O.

[0056] Here, according to the results of the fluid simulation conducted in Example 2, similar to Example 1, the flow velocity in the direction of the rotation axis of the wind blowing near the edge portion 41 of the rotating polyhedron 4 was smaller in Example 2 compared to the conventional example. This is because, as mentioned above, a high flow straightening effect was obtained by the arrangement of the straightening plate 31.

[0057] Furthermore, the inventors of the present invention also conducted noise measurement tests using the scanning optical device 101 of Example 2. The test conditions were the same as those for the Conventional Example and Example 1. The results showed that Example 2 achieved the same level of acoustic energy (60%) as the scanning optical device 101 of Example 1. As mentioned above, this is because the arrangement of the rectifier plate 31 enhanced the rectification effect, thereby suppressing the generation of turbulence.

[0058] In Example 2, a flow straightening plate 31 consisting of multiple independent linear ribs is placed near the rotating polyhedron 4. This allows for the disruption of the annular circulating vortex flow generated by the air being drawn in while rising relative to the center of rotation. Furthermore, it is possible to suppress turbulence caused by the collision of air descending near the edge portion 41 of the rotating polyhedron 4 with vortex-shaped air being pushed upward by the corner portion 42 formed by the reflective surfaces 11 of the rotating polyhedron 4. In this way, both the disruption of vortex flow and the suppression of turbulence can be obtained. This makes it possible to realize an optical scanning device with improved quietness.

[0059] As described above, according to Example 2, it is possible to improve quietness by achieving both the division of vortices generated by the rotation of the rotating polyhedron and the suppression of wind blowing onto the edge of the rotating polyhedron with an inexpensive configuration. [Examples]

[0060] [Explanation of laser beam printers] Figure 10 shows a schematic configuration of a laser beam printer as an example of an image forming apparatus. The laser beam printer 1000 (hereinafter referred to as printer 1000) includes a photosensitive drum 8 as a photoreceptor, a charging unit 1020, and a developing unit 1030. The photosensitive drum 8 is an image carrier on which an electrostatic latent image is formed. The charging unit 1020 uniformly charges the photosensitive drum 8. The scanning optical device 101, which is an exposure means, forms an electrostatic latent image by scanning laser light corresponding to the image data onto the photosensitive drum 8. The developing unit 1030 forms a toner image by developing the electrostatic latent image formed on the photosensitive drum 8 with toner. The toner image formed on the photosensitive drum 8 (on the image carrier) is transferred to a sheet P as a recording material supplied from a cassette 1040 by a transfer unit 1050, and the unfixed toner image transferred to the sheet P is fixed by a fuser 1060 and discharged into a tray 1070. The photosensitive drum 8, charging unit 1020, developing unit 1030, and transfer unit 1050 constitute the image forming unit. The printer 1000 is also equipped with a power supply unit 1080, which supplies power to the drive unit such as the motor and the control unit 5000. The control unit 5000 has a CPU (not shown) and controls the image forming operation by the image forming unit and the transport operation of the sheet P. After the printer 1000 finishes a print operation, it transitions to a standby state where it can immediately perform a print operation after a predetermined time has elapsed. After another predetermined time has elapsed, the printer 1000 transitions from the standby state to a sleep state, which is a low power consumption mode, in order to reduce power consumption while in standby mode. The printer 1000 has three states: a second mode, the sleep state and the standby state, and a first mode, the print state, and the control unit 5000 transitions to each of these states.

[0061] The scanning optical apparatus 101 has a cover member 20 on which the rectifier plate 21 of Example 1 or the rectifier plate 31 of Example 2 is provided. Note that the image forming apparatus to which the scanning optical apparatus 101 of the present invention can be applied is not limited to the configuration illustrated in Figure 10.

[0062] As described above, according to Example 3, it is possible to improve quietness by achieving both the division of vortices generated by the rotation of the rotating polyhedron and the suppression of wind blowing onto the edge of the rotating polyhedron with an inexpensive configuration.

[0063] This embodiment includes the following configuration. (Composition 1) A light source that emits a light beam, A rotating multifaceted mirror that deflects and scans the light beam emitted from the aforementioned light source, A scanning lens that forms an image of the light beam deflected and scanned by the rotating multifaceted mirror onto the surface to be scanned, An optical box having an opening, housing the light source, the rotating polyhedron mirror, and the scanning lens, A cover that covers the opening, A scanning optical apparatus comprising: The cover, when covering the opening, has a plurality of ribs formed on the surface of the cover facing the rotating polyhedron mirror, which protrude from the facing surface toward the rotating polyhedron mirror. The plurality of ribs are arranged such that they have rotational symmetry with respect to the rotation axis of the rotating polyhedron when viewed in the direction of the rotation axis of the rotating polyhedron, and extend from the rotation axis toward the circumscribed circle of the rotating polyhedron, and are the same number as the plurality of ribs, and are positioned at a predetermined distance upstream or downstream in the rotational direction of the rotating polyhedron with respect to a plurality of imaginary lines having rotational symmetry with respect to the rotation axis. Each of the ribs, when viewed in the direction of the rotation axis, has an elongated shape parallel to the corresponding imaginary line, with one end of the rib in the longitudinal direction positioned outside the circumscribed circle and the other end in the longitudinal direction positioned inside the circumscribed circle. A scanning optical device characterized by the following features. (Configuration 2) The aforementioned end is inclined from upstream to downstream in the direction of rotation when viewed in the direction of the rotation axis. A scanning optical apparatus according to configuration 1, characterized by the features described above. (Composition 3) The aforementioned inclination is formed by a curved surface. The scanning optical apparatus according to configuration 2, characterized in that it is a scanning optical apparatus. (Composition 4) The other end is formed by a curved surface such that, when viewed in the direction of the rotation axis, it forms a part of the circumference of a virtual circle centered on the rotation axis. A scanning optical apparatus according to any one of configurations 1 to 3, characterized by the above. (Composition 5) The rotating polyhedron has a plurality of reflective surfaces that reflect the light beam, The number of the aforementioned multiple ribs is equal to the number of the aforementioned multiple reflective surfaces. A scanning optical apparatus according to any one of configurations 1 to 4, characterized by the above. (Composition 6) When the rotating polyhedron has an even number of reflective surfaces that reflect the light beam, The plurality of ribs are arranged point-symmetrically with respect to the axis of rotation. A scanning optical apparatus according to any one of configurations 1 to 5, characterized by the above. (Composition 7) The rotating polyhedron mirror has a top surface facing the opposing surface of the cover when the cover covers the opening, The plurality of ribs are arranged between the opposing surface and the top surface. A scanning optical apparatus according to any one of configurations 1 to 6, characterized by the above. (Composition 8) An image forming apparatus that forms an image on a recording material, A photoreceptor having the scanning surface, A scanning optical apparatus as described in any of configurations 1 to 7, An image forming apparatus characterized by comprising: [Explanation of symbols]

[0064] 1. Semiconductor laser unit 4. Rotating polyhedron 7 Scanning lens 9 Optical box 20 Cover component 21 Rectifier plate 21-1~21-4 Ribs

Claims

1. A light source that emits a light beam, A rotating multifaceted mirror that deflects and scans the light beam emitted from the aforementioned light source, A scanning lens that forms an image of the light beam deflected and scanned by the rotating multifaceted mirror onto the surface to be scanned, An optical box having an opening, housing the light source, the rotating polyhedron mirror, and the scanning lens, A cover that covers the opening, A scanning optical apparatus comprising: The cover, when covering the opening, has a plurality of ribs formed on the surface of the cover facing the rotating polyhedron mirror, which protrude from the facing surface toward the rotating polyhedron mirror. The plurality of ribs are arranged such that they have rotational symmetry with respect to the rotation axis of the rotating polyhedron when viewed in the direction of the rotation axis of the rotating polyhedron, and extend from the rotation axis toward the circumscribed circle of the rotating polyhedron, and are the same number as the plurality of ribs, and are positioned at a predetermined distance upstream or downstream in the rotational direction of the rotating polyhedron with respect to a plurality of imaginary lines having rotational symmetry with respect to the rotation axis. Each of the ribs, when viewed in the direction of the rotation axis, has an elongated shape parallel to the corresponding imaginary line, with one end of the rib in the longitudinal direction positioned outside the circumscribed circle and the other end in the longitudinal direction positioned inside the circumscribed circle. A scanning optical device characterized by the following features.

2. The aforementioned end is inclined from upstream to downstream in the direction of rotation when viewed in the direction of the rotation axis. The scanning optical apparatus according to feature 1.

3. The aforementioned inclination is formed by a curved surface. The scanning optical apparatus according to claim 2.

4. The other end is formed by a curved surface such that, when viewed in the direction of the rotation axis, it forms a part of the circumference of a virtual circle centered on the rotation axis. The scanning optical apparatus according to feature 1.

5. The rotating polyhedron has a plurality of reflective surfaces that reflect the light beam, The number of the aforementioned multiple ribs is equal to the number of the aforementioned multiple reflective surfaces. The scanning optical apparatus according to feature 1.

6. When the rotating polyhedron has an even number of reflective surfaces that reflect the light beam, The plurality of ribs are arranged point-symmetrically with respect to the axis of rotation. The scanning optical apparatus according to feature 1.

7. The rotating polyhedron mirror has a top surface facing the opposing surface of the cover when the cover covers the opening, The plurality of ribs are arranged between the opposing surface and the top surface. The scanning optical apparatus according to feature 1.

8. An image forming apparatus that forms an image on a recording material, A photoreceptor having the scanning surface, A scanning optical apparatus according to any one of claims 1 to 7, An image forming apparatus characterized by comprising:

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