Optical scanning apparatus and image forming apparatus including optical scanning apparatus

At the time of forming an image, a rise in a temperature of a drive motor generates distortion in a bottom of a optical box of an optical scanning apparatus. If an opening is formed on the optical box to release heat, the optical box becomes easily distorted. To solve such a problem, according to the present invention, the optical scanning apparatus includes a rib which crosses over the opening formed at the bottom of the optical box.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to an optical scanning apparatus and an image forming apparatus including the optical scanning apparatus.

2. Description of the Related Art

An image forming apparatus employing an electrophotographic method, such as a laser beam printer or a copying machine, includes an optical scanning apparatus which emits a light beam for exposing a photosensitive member. The image forming apparatus forms an electrostatic latent image on the photosensitive member using the light beam emitted from the optical scanning apparatus. The image forming apparatus then develops the electrostatic latent image using toner and forms an image.

FIG. 13is a perspective view illustrating the optical scanning apparatus. Referring toFIG. 13, the light beam emitted from a light source1301is deflected by a reflection surface of a polygon mirror1302. The light beam deflected by the polygon mirror1302then passes through fθ lenses1303and1304, is reflected by a reflection mirror1305, and reaches a surface of the photosensitive member. Optical members such as the polygon mirror1302, the fθ lenses1303and1304, and the reflection mirror1305are mounted on a housing1306of the optical scanning apparatus.

When the image forming apparatus forms an image, the polygon mirror1302is rotationally driven by a drive motor. In general, the drive motor rotates at high speed, i.e., at 20,000 rpm to 40,000 rpm, so that temperature of the drive motor rises by 15° C. or more after a few minutes from starting to be driven. Temperature distribution is thus generated inside the housing1306due to heat generated by the drive motor becoming driven. The heat distribution causes uneven deformation of the housing1306to generate distortion.

In particular, since an amount of rise in temperature is greater in a portion where the drive motor is disposed as compared to other portions, an amount of deformation of the portion where the drive motor is disposed becomes relatively larger. As a result, the housing1306sags at a bottom to be basin-like shape with the drive motor as a center, as illustrated inFIG. 14. Referring toFIG. 14, the amount of distortion is exaggerated as compared to an actual amount of distortion so that deformation is easily recognizable. Since the deformation changes orientation of the optical members mounted on the housing1306from the desired orientation, an optical path of the light beam is changed from the desired optical path. Quality of the formed image is thus deteriorated.

In response to the above problem, Japanese Patent Application Laid-Open No. 2009-198890 discusses an optical scanning apparatus in which an opening is formed in the vicinity of the drive motor to allow the housing to be capable of ventilation between inside and outside the housing. By forming an opening, the heat inside the housing is released, so that deformation of the housing can be suppressed.

The optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. 2009-198890 is capable of reducing heat deformation by forming the opening. However, strength (i.e., rigidity) of a peripheral portion of the opening is lowered by forming the opening.

Referring toFIG. 13, ribs (e.g., ribs1307) which are reinforcement members are disposed in the housing of the optical scanning apparatus to increase the rigidity. Since the ribs are not disposed in a periphery of the opening in the optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. 2009-198890, the rigidity of the periphery of the opening is reduced.

SUMMARY OF THE INVENTION

According to an aspect of the present disclosure, An optical scanning apparatus comprising: a light source to emit a light beam; a rotational polygon mirror configured to deflect the light beam so the deflected light beam scans the a photosensitive member; a motor configured to rotationally drive the rotational polygonal mirror; and an optical box in which the rotational polygon mirror and the drive motor are disposed, wherein the optical box includes an opening and a connecting member configured to cross over the opening.

Further features and aspects will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings.

DESCRIPTION OF THE EMBODIMENTS

Various exemplary embodiments, features, and aspects of the disclosure will be described in detail below with reference to the drawings.

The first exemplary embodiment will be described below.FIG. 1is a schematic cross-sectional view illustrating a main portion of an electrophotographic image forming apparatus according to the present exemplary embodiment. Referring toFIG. 1, the image forming apparatus includes a sheet feed unit101which feeds sheets, an image forming unit102Y which forms a yellow toner image, an image forming unit102M which forms a magenta toner image, an image forming unit102C which forms a cyan toner image, and an image forming unit102Bk which forms a black toner image. Since the component of each image forming unit is the same, the configuration of the image forming unit will be described below using image forming unit102Y. The image forming unit102Y includes a photosensitive drum107Y, i.e., a photosensitive member, a charging apparatus108Y, and a developing apparatus109Y.

When the image forming apparatus forms an image, the charging apparatus108Y charges a surface of the photosensitive drum107Y. An optical scanning apparatus103to be described below then exposes the charged photosensitive drum107Y, so that the electrostatic latent image is formed on the photosensitive drum107Y. The electrostatic latent image is made a visible image (developed) by yellow toner supplied from the developing apparatus109Y.

Each of the image forming units102M,102C, and102Bk similarly includes photosensitive drums107M,107C, and107Bk, charging apparatuses108M,108C, and108Bk, and developing apparatuses109M,109C, and109Bk respectively. Functions of each of the elements are similar to those of the elements included in the image forming unit102Y.

The toner image formed on the photosensitive drum in each image forming unit is transferred from the photosensitive drum to an intermediate transfer belt105at a primary transfer portions (i.e., Ty, Tm, Tc, and Tbk). The toner images transferred to the intermediate transfer belt105are then collectively transferred to a recording sheet conveyed from the sheet feed unit101to a secondary transfer unit T2. The recording sheet on which the toner images are transferred is conveyed to a fixing apparatus106which heat-fixes the toner image on the recording sheet106. The recording sheet on which the fixing apparatus106has performed the fixing process is discharged to outside the image forming apparatus.

Next, the optical scanning apparatus will be described below. The optical scanning apparatus103exposes the photosensitive drums107Y and107M included in the image forming units102Y and102M, and an optical scanning apparatus104exposes the photosensitive drums107C and107Bk included in the image forming units102C and102Bk. Each photosensitive drum is exposed to the light beam, so that the electrostatic latent image is formed on the surface thereof.

Since the optical scanning apparatuses103and104are similarly configured, the optical scanning apparatus103will be described below as an example.

FIG. 2Ais a main scan cross-sectional view illustrating the optical path of the optical scanning apparatus103illustrated inFIG. 1, expanded on one plane. The main scan cross-section is a plane to which a rotational shaft of the drive motor that drives a polygon mirror to be described below is a normal.

Referring toFIG. 2A, the optical scanning apparatus103includes a light source201for emitting the light beam to which the photosensitive drum107M is exposed. The light beam (i.e., a laser light) emitted from the light source201is converted to a parallel light flux by a collimator lens202and becomes convergent light by a cylindrical lens203disposed immediately after the collimator lens202. The cylindrical lens203has refractive power to converge the light flux in a direction corresponding to a sub-scanning direction of the photosensitive drum107M (i.e., a rotational direction of the photosensitive drum107M). The light beam passing through the cylindrical lens203is formed into a predetermined shape by a diaphragm204and linearly focused on the reflection surface of a polygon mirror205, i.e., a rotational polygon mirror.

FIG. 2Bis a schematic cross-sectional view of the optical scanning apparatus103. Referring toFIG. 2B, the polygon mirror205is rotatably driven by a drive motor218. The light beam emitted from the light source201is deflected by the rotating polygon mirror205and is thus converted to a scanning light beam (a deflected light beam) which is caused to scan (i.e., moves on) the photosensitive drum107M in a predetermined direction (i.e., in a direction of an arrow M′). The scanning light beam passes through an fθ lens206, i.e., one of optical members, is reflected by a reflection mirror214, and then passes through an fθ lens207. A reflection mirror215guides the scanning light beam which passed through the fθ lens207to the photosensitive drum107M. The scanning light beam which has passed through the fθ lenses206and207moves on the photosensitive drum107M in a predetermined direction at constant speed.

Further, the optical scanning apparatus103includes a light source208which emits the light beam to which the photosensitive drum107Y is exposed. The light beam emitted from the light source208is converted to a parallel light flux by a collimator lens209and becomes convergent light by a cylindrical lens210disposed immediately after the collimator lens209. The cylindrical lens210has the refractive power to converge the light flux in the direction corresponding to the sub-scanning direction of the photosensitive drum107Y (i.e., the rotational direction of the photosensitive drum107Y). The light beam passing through the cylindrical lens210is formed into a predetermined shape by a diaphragm211and linearly-focused on the reflection surface of the polygon mirror205, i.e., the rotational polygon mirror.

Referring toFIG. 2B, the light beam emitted from the light source208is deflected by the rotating polygon mirror205and is thus converted to the scanning light beam which is caused to scan (i.e., moves on) the photosensitive drum107Y in a predetermined direction (i.e., in a direction of an arrow Y′). The scanning light beam passes through an fθ lens212, i.e., an optical member, is reflected by a reflection mirror216, and then passes through an fθ lens213. A reflection mirror217guides the scanning light beam which passed through the fθ lens213to the photosensitive drum107Y. The scanning light beam which has passed through the fθ lenses206and207moves on the photosensitive drum107M in a predetermined direction at constant speed.

The polygon mirror205, the drive motor218, the various lenses, and the reflection mirrors are contained inside a housing219(an optical box). The housing219is formed of a material which has been reinforced by mixing glass fiber in polyphenylene ether (PPE) and polystyrene (PS) resin.

As described above, the temperature in the vicinity of the polygon mirror205rises by 15° C. or more after a few minutes from when the drive motor has started rotating. Since the housing219is formed of resin, it is easily thermally-deformed. Particularly inside the optical scanning apparatus, the optical members such as the polygon mirror205, various lenses, and the reflection mirrors are contained, so that the heat generated from the drive motor218is not uniformly diffused in the housing219. As a result, when the image forming apparatus forms an image, heat distribution is generated in the housing219.

In particular, the amount of a rise in temperature becomes greater in the peripheral portion of the polygon mirror205as compared to portions other than the peripheral portion (i.e., outside the peripheral portion). An amount of thermal deformation thus relatively increases in the peripheral portion, and basin-like shape deformation is generated in the bottom of the housing219as illustrated inFIG. 14.

If the bottom becomes deformed to be basin-like shape, relative positional relations between the optical members become deformed, so that the optical path of the light beam is changed. As a result, the light beam is not focused on a desired position on the photosensitive drum. For example, the orientations of the reflection mirrors214and216greatly affect the optical path. If an angle in which each of the reflection mirrors214and216is positioned is changed by several minutes, an image forming position of the light beam on the photosensitive drum becomes displaced in the sub-scanning direction by 40 to 50 μm.

When the image forming apparatus forms the image by superimposing four color toner images, the above-described displacement of the image forming position of the light beam is visualized as color mis-registration and causes image quality deterioration. In particular, according to the present exemplary embodiment, the image forming apparatus employs the optical scanning apparatus that causes a plurality of light beams to scan in two directions opposing each other across the polygon mirror205. The deformation of the housing219causes an irradiation position to be changed symmetrically. A relative amount of color mis-registration thus doubles to 80 to 100 μm.

According to the present exemplary embodiment, an opening is formed in the housing219of the optical scanning apparatus to release the heat inside the housing219generated by the temperature rise caused by the polygon mirror205. Further, according to the present exemplary embodiment, reinforcement unit (reinforcement member, i.e., a connecting unit (member)) is disposed in the optical scanning apparatus for securing rigidity in the peripheral portion of the opening which has been reduced by forming the opening.

The opening will be described below.FIG. 3Ais a perspective view illustrating the housing219of the optical scanning apparatus described with reference toFIGS. 2A and 2B.FIG. 3Bis an enlarged view illustrating the periphery of the polygon mirror205illustrated inFIG. 3A.FIG. 3Cis an enlarged view illustrating the periphery of the position where the polygon mirror205was disposed in a state that the polygon mirror205has been removed.FIGS. 3A,3B, and3C illustrate only the polygon mirror205disposed in the housing219. However, the lenses and the reflection mirrors above described are actually disposed in the housing219. Further,FIG. 4is a cross-sectional view of the peripheral portion of the polygon mirror205.

Referring toFIG. 3B, the polygon mirror205and the drive motor218are mounted on a substrate301on which an integrated circuit (IC) for driving the drive motor218is mounted. The drive motor218is disposed under the polygon mirror205inFIG. 3B. At the time of assembling the optical scanning apparatus, the substrate301is mounted on bearing surfaces306,307,308, and309disposed on the housing219as illustrated inFIG. 3C, and fixed on the bearing surfaces by screws302,303,304, and305illustrated inFIG. 3B.

Referring toFIG. 3C, an opening H1is formed on the bottom of the housing219. The substrate301is screw-fixed to the bearing surfaces306,307,308, and309on the housing219, and a bearing218aof the drive motor218(refer toFIG. 4) is inserted into the opening H1. The bearing218aof the drive motor218is not engaged with the opening H1along an entire periphery in a rotational direction of the shaft in the bearing218a. There is a section along the entire circumference direction of the bearing218ain which a portion of the bearing218ais not in contact with the housing219. Specifically, the opening H1in the housing219is shaped with respect to the shape of the bearing218aso that, when the bearing218aof the drive motor218is seated (inserted) into the opening H1in the housing219, a gap H2is formed. The gap H2which is capable of ventilation is formed in at least a portion between the housing219and the bearing218a.

As a result, air inside the housing219is released to outside the housing219, and the air outside the housing219enters the housing219through the gap H2formed between the housing219and the bearing218a. The heat inside the housing219is thus released by the air inside the housing219being released to the outside. Further, the housing219and the optical members disposed inside the housing219are cooled by the air outside the housing219(i.e., the air which is relatively cooler as compared to inside the housing219) entering the housing219. Furthermore, an air layer is formed between the bearing218aand an edge of the opening H1by having the gap H2, so that it becomes difficult for the heat to be transferred from the bearing218ato the edge of the opening H1(i.e., the bottom of the housing219). It thus prevents the housing219to be locally deformed due to heat, and distortion of the housing219can be reduced.

According to the present exemplary embodiment, the opening H1is formed in the vicinity of the drive motor218. However, the location of the opening H1is not limited to the above. A similar result as described above can be expected by forming the opening H1in a location corresponding to the area in which the temperature becomes relatively high in the housing219.

According to the present exemplary embodiment, the bearing218aof the drive motor218is inserted into the opening H1, so that the temperature around the opening greatly rises as compared to other areas in the housing219. Since the edge portion of the opening H1is a free end, it can be easily deformed by heat. Basin-like shape deformation may thus be generated in the housing219as illustrated inFIG. 14when the image forming apparatus performs an image formation process.

To solve such a problem, according to the present exemplary embodiment, the rib, i.e., the reinforcement unit, is disposed in the optical scanning apparatus to secure the rigidity (i.e., strength) of the peripheral portion of the opening H1. As illustrated inFIG. 3C, ribs220aand220bconnect the edges of the opening H1in such a way the ribs220aand220bcross over the opening H1.

The reinforcement units will be described in detail below with reference toFIGS. 5A and 5B.FIG. 5Ais a perspective view illustrating an exterior portion of the housing219of the optical scanning apparatus. The ribs220aand220b, i.e., the reinforcement units, are stood (vertically-stood) from the external of the housing219(i.e., the back surface of the surface on which the drive motor is disposed inside the housing219).

FIG. 5Billustrates only the ribs220aand220bas extracted from the exterior portion of the optical scanning apparatus illustrated inFIG. 5A. Referring toFIG. 5B, when the opening H1is viewed from a direction of the rotational shaft of the drive motor218, each of the ribs220aand220bcrosses (or longitudinally traverses) the opening H1. Further, the ribs220aand220bintersect on an extended direction from the rotational shaft of the drive motor218. According to the present exemplary embodiment, a plurality of ribs220aand220bis disposed. However, there may be only one rib220a, or three or more ribs. For example, as illustrated inFIG. 6, the ribs may be shaped to radiate in a Y-shape from the center of the opening of the drive motor218along the bottom surface of the housing219. Referring toFIG. 6, a rib601is radially-extended from the center of the opening H1along the bottom surface of the housing219.

Referring toFIG. 5B, the rib220ais a reinforcement unit which connects a point W and a point X on the edge portion of the opening H1to maintain a relative positional relation between the points W and X. Further, the rib220bis a reinforcement unit which connects a point Y and a point Z on the edge portion of the opening H1. The points W, X, Y, and Z correspond to the rim (edge) of the opening H1.

An engagement portion to which the bearing218ais to be engaged will be described below with reference toFIGS. 3C,4, and7. Referring toFIG. 3C, a notch portion (i.e., a step portion) which is an engagement unit310to which the bearing218abecomes engaged is formed on the ribs220aand220b(hereinafter referred to as a cross rib220).

FIG. 7illustrates the opening H1and the cross rib220as viewed from the direction of the rotational shaft of the drive motor218. Referring toFIG. 7, a step of width D is formed on each of the ribs220aand220b. The bearing218aof the drive motor218, which is cylindrically-shaped and having a diameter D, is formed of material such as brass. The bearing218aof the drive motor218is thus engaged with the engagement unit. At the time of assembling the optical scanning apparatus, the bearing218ais engaged with the engagement unit310to position the drive motor218, and the substrate301is then fixed by the screws302,303,304, and305.

Dimensions of the ribs will be described below with reference toFIG. 4. For ease of description,FIG. 4illustrates only the rib220bamong the cross rib220, and the rib220ais omitted. The cross rib220drops in height by one step from a height h1to a height h2at midway extending from the edge of the opening H1on the housing219toward the center of the opening H1. The bearing218athus becomes engaged with the difference in height. According to the present exemplary embodiment, h1is approximately 5 mm, h2is approximately 2.5 mm, and a width W of the rib is approximately 2 mm.

The effect acquired according to the present exemplary embodiment will be described below.FIGS. 8A and 8Billustrate the optical scanning apparatus which is a comparison example with respect to the present exemplary embodiment, and is the same as the optical scanning apparatus illustrated inFIG. 13.FIG. 8Bis an enlarged view illustrating an installation location of the polygon mirror. In the comparison example, an opening H3, wherein the diameter of the opening H1is greater than the diameter of the opening H3(H1>H3), is formed on the bottom of the housing1306. The bearing of drive motor contacts with the housing1306along an entire periphery, so that the gap H2is not formed due to the housing1306and the bearing of the drive motor, unlike the present exemplary embodiment. As a result, ventilation between inside the housing1306and outside the housing1306is impossible.

FIG. 9illustrates a difference of the temperature distribution in the housing219between the present exemplary embodiment and the comparison example. More specifically,FIG. 9illustrates the rise in the temperature during 10 minutes from when the drive motor has started to be driven, at each of points A and B illustrated in the cross-sectional view ofFIG. 4. A distance from the center of the drive motor to point A is approximately 10 mm, and a distance from the center of the drive motor to point B is approximately 22 mm.

Referring toFIG. 9, an experiment result acquired according to the present exemplary embodiment is indicated by a solid line, and an experiment result acquired according to the comparison example is indicated by a broken line. A temperature gradient between point A and point B is approximately 1° C. lower in the result acquired according to the present exemplary embodiment as compared to the result acquired according to the comparison example. It can thus be determined that it is harder to generate an uneven linear expansion which distorts the housing according to the present exemplary embodiment as compared to the comparison example.

FIG. 10illustrates results of analyzing static strength of the peripheral portions of the openings H1and H3according to the present exemplary embodiment and the comparison example. More specifically,FIG. 10illustrates the amounts of deformation in the edges of the openings H1and H3when a unit load in the direction of the rotational shaft of the drive motor is applied to the peripheries of the openings H1and H3. Referring toFIG. 10, if the amount of deformation is set as 100% for the comparison example in which there is no cross rib220, the amount of deformation according to the present exemplary embodiment remains at approximately 88%. It can be understood that the strength is improved by 12% according to the present exemplary embodiment as compared to the comparison example. Further, if a cross-sectional area of the rib is increased, the strength of the periphery portion of the opening H1can be increased.

FIG. 11illustrates results of measuring the amounts of deformation (i.e., an inclination) of a portion indicated by a bold arrow (i.e., a deformation measurement point) illustrated in the cross-sectional view ofFIG. 4. More specifically,FIG. 11illustrates how an angle of a plane at the measurement point has changed in 10 minutes from when the motor has started to be driven. The amount of change according to the conventional example is approximately 180 seconds. In contrast, according to the present exemplary embodiment, the amount of change is reduced to 100 seconds, so that the deformation can be reduced by 45% according to the present exemplary embodiment as compared to the comparison example.

As described above, the opening H1is formed to be capable of ventilation between inside and outside the housing219. The rigidity of the housing219in the peripheral portion of the opening H1which has been lowered by formation of the opening H1is secured by disposing the ribs which cross over or traverse the opening H1. Generation of heat deformation of the housing219when the drive motor218is driven can thus be reduced.

The second exemplary embodiment will be described below. According to the first exemplary embodiment, the opening H1, which is capable of ventilation, is formed between the housing219and the bearing218ain the optical scanning apparatus. However, according to the first exemplary embodiment, dust may enter the housing219via the opening H1. According to the present exemplary embodiment, the opening H1is closed by a dust preventing seal to improve dust prevention as compared to the first exemplary embodiment.

FIGS. 12A and 12Bare perspective views illustrating the exterior portion of the housing used in the optical scanning apparatus according to the present exemplary embodiment. The opening H1, and the ribs220aand220b, which are disposed to cross over the opening H1, are similar to the first exemplary embodiment. However, a rib1201disposed as a reinforcement unit surrounding the opening is different from the first exemplary embodiment. The height of the rib1201from the bottom surface of the housing is higher than the heights of the ribs220aand220bfrom the bottom surface of the housing. Further, according to the present exemplary embodiment, the rib1201surrounding the opening and rib220a, and the rib1201and the rib220bare connected respectively in the optical scanning apparatus. The strength of the peripheral portion of the opening is thus higher as compared to the first exemplary embodiment owing to the rib1201.

FIG. 12Billustrates the optical scanning apparatus in which a dust preventing seal1202, i.e., a dust prevention member, is attached. The opening H1formed by the housing219and the bearing218ais covered by attaching the dust preventing seal1202, so that the dust entering the housing219can be reduced. Further, the dust preventing seal1202is formed of a material whose heat-transfer coefficient is higher than the resin used in forming the housing219. As a result, the heat inside the housing is more easily released to the outside as compared to a configuration in which the opening H1is covered with resin. According to the present exemplary embodiment, a thickness of the dust preventing seal1202may be thinner than the thickness of the bottom of the housing219. The heat inside the housing219can thus be more easily released to outside the housing219via the dust preventing seal1202as compared to the configuration in which there is no opening formed on the bottom surface of the housing219.

As described above, the rigidity of the peripheral portion of the opening H1can be increased by disposing the rib1201, which connects to the ribs220aand220band surrounds the opening H1. Further, the dust can be prevented from entering the housing by attaching the dust preventing seal1202on the rib1201whose height from the bottom surface of the housing is higher than that of the ribs220aand220b.

The shape of the ribs are not limited to the shapes illustrated inFIG. 3Caccording to the first exemplary embodiment andFIG. 12Aaccording to the present exemplary embodiment. Another shape of the rib will be described below with reference toFIGS. 15A and 15B.FIG. 15Ais a perspective view illustrating the area surrounding the opening H1in the housing219.FIG. 15Bis a top view ofFIG. 15A.FIG. 15Cis a cross-sectional view taken along a line A-A illustrated inFIG. 15B. Referring to15A, ribs1501,1502,1503, and1504extend from each of edges W′, X′, Y′, and Z′ of the opening H1to the center portion of the opening H1. Each of the ribs1501,1502,1503, and1504is connected to a circular rib1505formed at the center portion of the opening H1. The shape of the rib is thus not limited to the cross-shaped rib as illustrated inFIG. 3C.

Further, referring toFIG. 15C, a reinforcement unit1506is disposed on the exterior portion of the housing219to surround the opening H1, to increase the rigidity of the opening H1. The reinforcement unit1506and each of the ribs1501,1502,1503, and1504may be connected as illustrated inFIG. 12Ato increase the rigidity.

OTHER EMBODIMENTS

This application claims priority from Japanese Patent Application No. 2011-094156 filed Apr. 20, 2011 and Japanese Patent Application No. 2012-029865 filed Feb. 14, 2012, each of which is hereby incorporated by reference herein in their entirety.