For example, an image forming apparatus such as a color copying machine or a printer includes an exposing device. The exposing device includes a laser diode as a light source, a polygon mirror, a deflection lens (an fθ lens), and a reflection mirror. The exposing device may be referred to as a laser scanning unit or an optical scanning device. For example, as disclosed in U.S. Pat. No. 6,700,687 B1, a light source, a polygon mirror, a deflection lens, a reflection mirror, and the like are housed in a casing. A laser beam emitted from the light source is transmitted through the deflection lens via the polygon mirror and reflected by the reflection mirror to travel to a photoconductive drum.
The reflection mirror has a body square-bar-shaped in section and a reflection layer formed by depositing aluminum on the body. The mirror has a first surface on which the reflection layer is formed, second and third surfaces perpendicular to the first surface, and a fourth surface on the opposite side of the reflection layer. One end in an axial direction (a longitudinal direction) of the mirror is supported by one mirror supporting section provided in the casing. The other end in the axial direction of the mirror is supported by the other mirror supporting section provided in the casing. The exposing device of this type needs to reflect the laser beam at a predetermined angle with respect to the photoconductive drum. Therefore, the exposing device includes an adjusting mechanism for adjusting an angle of the mirror.
For example, the exposing device in the past, one mirror supporting section for supporting one end of the mirror has a positioning protrusion that comes into contact with the first surface of the mirror and an adjusting screw provided beside the positioning protrusion. A distal end of the adjusting screw is in contact with the first surface. A first flat spring is in contact with the fourth surface of the mirror with elastic energy stored therein. The first flat spring urges an end of the mirror toward the positioning protrusion. A positioning section that comes into contact with the second surface of the mirror is provided in one mirror supporting section. A second flat spring is in contact with the third surface of the mirror with elastic energy stored therein. The second flat spring urges one end of the mirror toward the positioning section. The other end of the mirror is elastically supported by a flat spring and a positioning protrusion provided in the other mirror supporting section.
To adjust an angle of the mirror, a projecting amount of the adjusting screw with respect to the mirror is changed by rotating the adjusting screw. For example, when the projecting amount of the adjusting screw is increased, while bending of the first flat spring increases, the mirror rotates in a first direction with the respective positioning protrusions as fulcrums. When the projecting amount of the adjusting screw is decreased, the mirror rotates in a second direction with the positioning protrusions as fulcrums with the repulsion of the first flat spring.
In the adjusting mechanism in the past, it is difficult to keep the balance of loads between the flat springs and the adjusting screw. For example, if the repulsion of the flat spring is too strong, when the projecting amount of the adjusting screw is increased, force larger than necessary force is applied to the mirror. This causes breakage of the mirror and shift of a position of the mirror from a predetermined position. Conversely, if the repulsion of the flat spring is too weak, when the projecting amount of the adjusting screw is reduced, the flat spring cannot push back the mirror. Therefore, the mirror cannot be rotated in a desired direction. Further, if the repulsion of the flat spring is weak, for example, a position of the mirror is not stabilized and the mirror moves when impact is applied to the mirror. In particular, when “permanent set in fatigue” due to aged deterioration of the flat spring is taken into account, it is difficult to properly keep the balance of loads between the repulsion of the flat spring and the adjusting screw.
Both the ends of the mirror are supported by a supporting section of the casing of the exposing device. Besides the mirror, a light source, a polygon mirror, a polygon motors a motor driver, and the like are housed in the casing. These act as heat generation sources during the operation of the exposing device. The casing of the exposing device may be affected by heat generated from peripheral apparatuses.
A casing of synthetic resin can be easily molded in a desired shape, light in weight, and low in cost compared with a casing of metal. However, the casing of synthetic resin has small thermal conductivity compared with the casing of metal. Therefore, a thermal expansion amount near a mirror supporting section close to a heat source and a thermal expansion amount near a mirror supporting section distant from the heat source may be different. This causes an adverse effect on a quality of an image in an exposing device including plural mirrors that are used in a color copying machine.
For example, in a mirror supporting section in a position close to a light source, since a thermal expansion amount is large, positional deviation of a mirror is large. An amount of shift of a beam reflected by the mirror is large. On the other hand, in a mirror supporting section in a position distant from the light source, since a thermal expansion amount is small, positional deviation of a mirror is small. An amount of shift of a beam reflected by the mirror is small. As a result, the amount of shift of the beams is different for each of the mirrors. This causes color drift on a photoconductive member. In that case, adjustment work is necessary for measures against the color drift. Therefore, for example, printing is unfavorably interrupted.
If the casing is integrally molded by using a material having high thermal conductivity such as an aluminum alloy, the thermal expansion difference between the mirror supporting sections can be reduced. However, the casing of metal is high in cost compared with the casing of synthetic resin. As another measure, it is also conceivable to fix a press-formed sheet metal member to the casing of synthetic resin. This sheet metal member is formed in a predetermined shape by pressing a metal plate. A mirror supporting member is provided in the sheet metal member. However, in the sheet metal member in which a bent section and the like are formed by pressing, it is difficult to highly accurately form the mirror supporting section in terms of a position and a shape.