JP2013186335A - Optical scanner, and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical scanner capable of contributing to reduction of the thickness of an optical housing and reduction of the size of an image forming apparatus as well, reducing fluctuation in an optical function on an image carrier (especially, a scan line position), and contributing to reduction of costs by eliminating the need of detection means and correction means for color shift correction that requires high costs.SOLUTION: An optical housing 101 includes a rib 205 that connects a side wall 201 on the +Y axis direction side and a side wall 202 on the -Y axis direction side, on the +X axis direction side and -X axis direction side thereof, respectively, with a Y-Z cross section therebetween. The height H of the rib as a thermal deformation mode regulating member is set such that a thermal deformation mode in the Y-Z cross section of the optical housing and a deformation mode in an X-Z cross section including an X-axis direction and a Y-axis direction that are associated with drive of a polygon scanner face opposite directions with each other on a sign of curvature.

Description

  The present invention relates to an optical scanning device for forming an electrostatic latent image on a surface to be scanned, a copying machine having the optical scanning device, a printer, a facsimile machine, a plotter, and an image forming apparatus such as a multifunction machine provided with at least one of them. About.

In electrophotographic image recording, an image forming apparatus using a laser is widely used. In this case, the image forming apparatus includes an optical scanning device, and scans laser light using an optical deflector (for example, a polygon mirror) in the axial direction of a photosensitive drum (hereinafter also referred to as “photosensitive drum”). In general, a method of rotating the photosensitive drum to form a latent image on the surface of the photosensitive drum is common.
2. Description of the Related Art In recent years, tandem image forming apparatuses having a plurality of photosensitive drums (usually four) have become widespread as image forming apparatuses have become more colorized and faster.
In the tandem system, the image forming apparatus tends to increase in size as the number of photosensitive drums increases, and downsizing including an optical scanning apparatus is required. In order to reduce the size of the optical scanning device (in particular, to reduce the thickness), it is effective to superimpose a plurality of optical paths of scanning light from the optical deflector toward each photosensitive drum.

In order to achieve thinning of the optical scanning device, it is necessary to devise the optical path layout (sub-scanning cross section) of the laser beam from the optical deflector to the surface of the photosensitive drum inside the device.
For example, Patent Document 1 proposes a method in which the optical path layout is a “two-stage configuration” in a configuration in which laser beams separated from each other by utilizing polarization characteristics are guided to different photosensitive drums.

In order to reduce the thickness of the optical scanning device, it is necessary to reduce the thickness of the optical housing that houses the optical element. When the optical housing is thinned, for example, the optical housing is thermally deformed due to the influence of heat generated by driving the optical deflector.
In addition, vibration characteristics such as a decrease in the natural frequency of the optical housing and an increase in the amplitude deteriorate. As a result, the relative positional relationship between the optical elements is shifted, and as a result, the optical characteristics (beam spot position, beam spot diameter, etc.) of the laser beam that scans the surface of the photosensitive drum deteriorate. As a result, the output image quality of the image forming apparatus deteriorates.

In the following, we will consider the problems when the optical housing is made thin.
[Thermal deformation of optical housing]
As a method for reducing the relative displacement between optical elements due to thermal deformation of the optical housing, a method described in Patent Document 2 is known. In this method, the coefficient of thermal expansion of the mounting portion of the cylindrical lens and the scanning lens system is set so as to cancel out the optical axis shift of the laser light from the laser unit accompanying the temperature change.
Patent Document 3 discloses a configuration in which a posture change accompanying a temperature change of a mirror disposed in an optical path between a deflecting unit and a surface to be scanned is corrected by an expansion / contraction member that expands and contracts by thermal expansion.
However, in the configurations described in Patent Documents 2 and 3, it is necessary to combine materials having different thermal expansion coefficients, and it is inevitable that the configuration of the optical housing is complicated and expensive.

In Patent Document 4, the optical axis of the light source unit is changed in the sub-scanning direction by driving a piezo element or a heating element based on the detection result of the light beam detecting means provided in the light beam exit of the deflection scanning device. The structure provided with the means to make is disclosed.
However, in the configuration described in Patent Document 4, since it is necessary to include a beam detection unit, a piezo element, and a heating element, high cost is likely to be caused.

[Vibration of optical housing]
In Patent Document 5, at least two attachment points serving as vibration nodes are spaced apart in the longitudinal direction of the optical component on the bottom wall of the sheet metal housing on which the optical component is mounted and the vibration of the component propagates. An optical scanning device is disclosed in which an optical component holding member that holds an optical component is attached to a mounting location, and the optical component holding member floats from the vibration node to the node with respect to the bottom wall. Yes.
However, even if it is going to employ | adopt the vibration suppression structure currently disclosed by patent document 5, it is not easy to set a part of a node in arbitrary positions on the design of an optical housing, and also on optical design, it is an optical element. It was extremely difficult to arrange the in accordance with the position of the node.
Patent Document 6 discloses a configuration that secures the rigidity of the optical housing and improves vibration resistance by providing a pair of ribs that connect the rear side wall and the front side wall of the optical housing.
That is, a strip-shaped plate material (rib) extending in the main scanning direction is connected between the rear side wall and the front side wall so as to suppress a decrease in strength at the longitudinal center portion of the housing orthogonal to the main scanning direction. .
Here, the height of the rib is recognized only from the viewpoint of increasing the mechanical strength (rigidity), and is set to the same height as the rear side wall and the front side wall.

For example, in the case of an optical scanning device that is not conscious of thinning as shown in FIG. 5 of Patent Document 3, thermal deformation is prevented by relatively freely configuring the shape of the bottom plate and side wall of the optical housing. It is possible to take measures to increase vibration resistance.
However, in the case of a thin optical scanning device, in order to secure the arrangement of optical elements and the optical path of the laser beam, as shown in FIG. 5, the optical housing 101 is composed of a flat bottom plate 206 and its peripheral portion (4 The side walls 201, 202, 203, and 204 formed perpendicular to the side) and ribs 205 disposed in the vicinity of the polygon mirror are required.
By providing ribs in the vicinity of the polygon mirror, the opening 207 through which the laser beam passes can be made small, and the influence of the opening can be minimized.
Even if ribs for preventing thermal deformation and improving vibration resistance are arranged at positions other than the vicinity of the polygon mirror, it is substantially possible to deploy effective ribs at positions where they do not interfere with the laser beam, as can be predicted from FIG. It is difficult to.

As described above, with the thinning of the optical scanning device, the optical performance (particularly, the scanning line position) deteriorates due to the thermal deformation of the optical housing and the deterioration of the vibration characteristics.
When the scanning line position fluctuates due to thermal deformation of the optical housing, color misregistration (image deterioration in which a plurality of toner images do not overlap) occurs in the output image by the image output device.
In order to avoid this, conventionally, a “color misregistration detection toner image” formed on the transfer belt is detected using a “color misregistration detection sensor”, and the writing start timing is corrected based on the detection result. Thus, the occurrence of color misregistration is reduced.
However, in the case of such countermeasures, since a “toner image for color misregistration detection” is formed, toner that is not used for the output image is consumed, increasing the environmental load. Also, the correction operation is often programmed to be executed when, for example, 100 images are output or when the temperature difference inside the apparatus becomes 5 ° C. or more.
For this reason, color misregistration may occur before the correction operation is executed. Further, if the correction operation is frequently performed, the image output operation cannot be performed during that time, leading to a decrease in productivity (number of output sheets per unit time) as an image output machine.

  The present invention was devised in view of such a current situation, and can contribute to a reduction in the thickness of the optical housing and, in turn, to a reduction in the size of the image forming apparatus, as well as the optical performance (especially the scanning line position) on the image carrier. The main object of the present invention is to provide an optical scanning device that can reduce fluctuations and contribute to cost reduction by eliminating expensive detection means and correction means for correcting color misregistration.

  In order to achieve the above object, the present invention provides a light source means, a deflection means for deflecting a laser beam emitted from the light source means, and a laser beam deflected by the deflection means as a light spot on a surface to be scanned. A scanning optical system that forms an image, the axial direction of the rotation axis of the deflecting unit is the Z-axis direction as the sub-scanning direction, the main scanning direction is the Y-axis direction, and the Z-axis direction and the Y-axis direction are When the orthogonal direction is the X axis direction, the scanning optical system is provided on both the + X axis direction side and the −X axis direction side across the YZ section including the Y axis direction and the Z axis direction, and the light source The means is arranged between the scanning optical system on the + X axis direction side and the −X axis direction side with respect to the X axis direction, and bends the optical path of the laser beam between the deflection means and the surface to be scanned. An even number of folding mirrors, and the light source means, the deflecting means, the scanning optical system and the folding mirror are provided. A housing for storing the return mirror is provided, and the housing extends in the Z-axis direction on the four sides of the bottom plate in a substantially rectangular shape parallel to the XY plane including the X-axis direction and the Y-axis direction. In the optical scanning device, the dimension of the casing in the X-axis direction is larger than the dimension in the Y-axis direction. In the optical scanning device, the casing has a + X-axis direction side and -X across the YZ section. Ribs that connect the side wall on the + Y axis direction side of the casing and the side wall on the −Y axis direction side of the casing on the axial direction side, respectively, in the YZ cross section of the casing accompanying the driving of the deflection means It has a thermal deformation mode regulating member that regulates the thermal deformation mode and the deformation mode in the XZ cross section including the X-axis direction and the Z-axis direction to be opposite to each other on the sign of curvature. Characterize

According to the present invention, by setting the conditions of the constituent elements of the casing, the thermal deformation mode of the casing accompanying the driving of the deflection unit is intentionally regulated so that the occurrence of the scanning line position fluctuation can be suppressed. Can do.
As a result, the optical housing can be reduced in thickness without causing an increase in cost, and consequently, the image forming apparatus can be reduced in size, and fluctuations in the optical performance (particularly the scanning line position) on the image carrier can be reduced. This also contributes to cost reduction by eliminating the need for expensive detection means and correction means for color misregistration correction. Further, the color image forming apparatus can contribute to providing a high-quality output image with little color misregistration.

1 is a schematic configuration diagram of an image forming apparatus according to a first embodiment of the present invention. It is a perspective view of the state which removed the cover member of the optical scanning device. It is a perspective view of the state where the cover member of the optical scanning device was attached. It is a schematic sectional drawing in the subscanning direction of an optical scanning device. It is a perspective view of the optical housing as a housing | casing. It is a perspective view of the polygon scanner as a deflection | deviation means. It is a schematic sectional drawing in the subscanning direction of the optical scanning device when not using an optical path separation means. It is a perspective view of the optical housing for simulation of thermal deformation analysis. It is a figure which shows the result of the heat transfer analysis of the optical housing in simulation. It is a figure which shows the result of the thermal stress analysis of the optical housing in simulation. It is a schematic diagram for demonstrating the scanning line position fluctuation | variation resulting from the thermal deformation of an optical housing. It is a schematic diagram for explaining the scanning line position variation on the photosensitive drum due to the thermal deformation of the optical housing. FIG. 6 is a characteristic diagram based on an experiment showing the relationship between the height of the rib and the amount of deformation; (a) is a characteristic diagram showing the relationship between the height of the rib and the amount of deformation of the optical housing; It is a characteristic view which shows the relationship with fluctuation. It is a schematic diagram which shows the definition of the deformation amount of an optical housing. It is a characteristic view based on the experiment which shows the influence of the height of the rib which acts on a primary resonant frequency. It is a perspective view of the optical housing in 2nd Embodiment (a rib is cut in the shape of an inverted triangle). FIG. 6 is a characteristic diagram based on an experiment showing the relationship between the cut depth D and the deformation amount, (a) is a characteristic diagram showing the relationship between the cut depth and the deformation amount of the optical housing, and (b) is a variation in the cut depth and scanning line position. It is a characteristic view which shows the relationship. It is a perspective view of the optical housing in 3rd Embodiment (a rib is cut into triangle shape). FIG. 6 is a characteristic diagram based on an experiment showing the relationship between the cut depth E and the deformation amount, (a) is a characteristic diagram showing the relationship between the cut depth and the deformation amount of the optical housing, and (b) is a variation in the cut depth and scanning line position. It is a characteristic view which shows the relationship. FIG. 10 is a characteristic diagram based on an experiment showing the relationship between the width W and the deformation amount in the fourth embodiment (adjusting the rib width), (a) is a characteristic diagram showing the relationship between the width and the deformation amount of the optical housing; b) is a characteristic diagram showing the relationship between width and scan line position fluctuation. It is a perspective view of the optical housing in 5th Embodiment (adjustment of the arrangement number of a rib). FIG. 6 is a characteristic diagram based on experiments showing the relationship between the number of ribs arranged and the amount of deformation; FIG. 5A is a characteristic diagram showing the relationship between the number of arrangement and the amount of deformation of the optical housing; FIG. It is a characteristic view which shows the relationship.

Embodiments of the present invention will be described below with reference to the drawings.
The first embodiment will be described with reference to FIGS. First, an outline of the configuration of an image forming apparatus capable of full color image formation according to the present embodiment will be described with reference to FIG.
As shown in FIG. 1, the image forming apparatus according to the present embodiment includes image forming apparatuses 15Y (yellow), 15M (magenta), 15C (cyan), and 15K (for four colors as image carrier units or process units. The image forming device 15 serving as each station is detachably attached to the device main body 80.
In addition to each image forming device 15, an optical scanning device 20 as an exposure unit capable of irradiating a laser beam, an intermediate transfer body unit 30, a paper feed unit 60, a fixing unit 50 as a fixing unit, and the like are provided.

The structures of the image forming devices 15Y, 15C, 15M, and 15K are the same except that the development colors are different. The yellow image forming device 15Y will be described as a representative. The photosensitive drum 9Y as an image carrier, the charging device 13Y for charging the photosensitive drum, and the cleaning device for removing the developer remaining on the photosensitive drum. 14Y and a developing device 16Y that develops the latent image formed on the photosensitive drum are integrally provided.
Each image forming apparatus is configured to be detachable from the apparatus main body in the opening / closing direction of an openable / closable face plate (not shown) provided on the operation surface side of the apparatus main body, that is, the rotation axis direction of the photosensitive drum.

The intermediate transfer body unit 30 includes an endless transfer belt 31 as an intermediate transfer body and four primary transfer rollers 35 as primary transfer means for transferring the toner images formed on the respective photosensitive drums 9 to the transfer belt 31. And a secondary transfer roller 36 as secondary transfer means for transferring the toner image transferred onto the transfer belt 31 onto a recording paper P as a recording medium.
The transfer belt 31 is wound around support rollers 32, 33, 34, and 37, and the support roller 32 faces the secondary transfer roller 36 across the transfer belt 31, and between the secondary transfer roller 36. A secondary transfer nip portion is formed.
A belt cleaning device 38 for cleaning the surface of the transfer belt 31 after transfer is provided at a portion facing the support roller 33.

The paper feed unit 60 has a configuration in which a bundle of recording paper P is brought into contact with the calling roller 61 by operating an arm 48 installed inside the paper feed cassette 41 by a lift motor (not shown) to raise the tray bottom plate 47. Have.
By rotating the calling roller 61, the bundle of recording papers P is shifted downstream in the paper feeding direction, and one sheet of the uppermost paper is separated by the paper feeding roller 62 and the reverse roller 63, and the conveying roller pair 64 and the registration roller pair 65 are separated. Transport to secondary transfer area.
The fixing unit 50 includes a fixing roller 51 and a pressure roller 52, and performs fixing by applying heat and pressure to the toner image on the recording paper P.

In the above configuration, first, in the first color yellow image forming device 15Y, after the photosensitive drum 9 is uniformly charged by the charging device 13, a latent image is formed by the laser light emitted from the optical scanning device 20, The latent image is visualized as a toner image by the developing device 16.
The toner image formed on the photosensitive drum 9 is transferred onto the transfer belt 31 by the action of the primary transfer roller 35. After completion of the primary transfer, the photosensitive drum 12 is cleaned by the cleaning device 14 to prepare for the next image formation.
The residual toner collected by the cleaning device 14 is stored in a waste toner collection bottle (not shown) installed in the take-out direction of the image forming device. The waste toner collection bottle is detachable from the apparatus main body so that it can be replaced when it is full.

A similar image forming process is performed in the image forming apparatuses 15M, 15C, and 15K to form toner images of the respective colors, which are sequentially superimposed and transferred onto the previously formed toner images.
On the other hand, the recording paper P is conveyed from the paper feed cassette 41 to the secondary transfer area, and the toner image formed on the transfer belt 31 is secondarily transferred to the recording paper P by the action of the secondary transfer roller 36.
The recording paper P to which the toner image has been transferred is conveyed to the fixing unit 50, where the toner image is fixed at the nip portion between the fixing roller 51 and the pressure roller 52 of the fixing unit 50, and discharged by the discharge roller pair 55. The paper is discharged to the tray 56.
The recording paper P is also set on the manual feed tray 59, and is similarly fed and transported by the paper feed roller 58, transferred and fixed, and discharged to the paper discharge tray 56.
The toner of each image forming device 15Y, 15M, 15C, 15K is consumed by the image formation. In response to this, new toner is supplied from the toner bottles 57Y, 57M, 57C, and 57K. At the time of toner supply, the toner bottles 57Y, 57M, 57C, and 57K are rotated, and the toner is conveyed to each image forming device through a pipe (not shown).

Hereinafter, the configuration of the optical scanning device 20 will be described in detail.
2 is a perspective view with the cover member 101-1 of FIG. 3 removed, FIG. 4 is a sub-scan sectional view, and FIG. 5 is a configuration of only the housing.
In the present embodiment, the rotational axis of the polygon scanner 214 as the deflecting unit, strictly speaking, the axial direction of the rotational axis of the polygon mirror 214-1 is the Z-axis direction (sub-scanning direction), and the longitudinal direction of each photosensitive drum 9 The (rotational axis direction) is defined as the Y-axis direction (main scanning direction), and the direction orthogonal to the Z-axis direction and the Y-axis direction (arrangement direction of the photosensitive drums 9) is defined as the X-axis direction.

As shown in FIG. 5, an optical housing 101 as a housing includes a bottom plate 206 having a substantially rectangular shape parallel to the XY plane including the X-axis direction and the Y-axis direction, and the four sides of the bottom plate 206 in the + Z-axis direction. The rear side wall 201, the front side wall 202, the right side wall 203, and the left side wall 204 as extending side walls, and a pair of ribs 205 right and 205 left connecting the rear side wall 201 and the front side wall 202 are configured.
By forming the ribs to be paired at an equal distance from the YZ cross section including the Y axis direction and the Z axis direction, the “thermal deformation mode” of the optical housing 101 in the XZ cross section including the X axis direction and the Z axis direction is achieved. This is preferable because it is symmetrical with respect to the YZ section. The deformation mode will be described later.
In FIG. 5, the outer dimensions of the optical housing 101 are a width m1 = 400 mm in the X-axis direction, a depth m2 = 320 mm in the Y-axis direction, and a height m3 = 34 mm in the Z-axis direction.
The thickness of the side wall and bottom plate 206 is 4 mm.

As shown in FIG. 2, a polygon scanner 214 is disposed in the central portion of the optical housing 101 in the X-axis direction, that is, in a region between a pair of ribs 205 right and 205 left. Two circular holes 201a are formed in the central portion of the rear side wall 201 in the left-right direction (X-axis direction), and the semiconductor laser 211 right and 211 left as light sources are inserted into these holes, respectively. .
Openings 207 right and 207 left are formed at the center of the Y axis direction (main scanning direction) of the ribs 205 right and 205 left, and serve as a laser beam exit window reflected from a deflection reflection surface of a polygon mirror (not shown). .

The laser beams 219C and 219K emitted from the semiconductor laser 211 right are converted into a parallel light beam, a weak divergent light beam, or a weak convergent light beam according to the characteristics of the subsequent optical system by the action of the coupling lens 212 right. The laser beams 219C and 219K emitted from the right of the coupling lens 212 are coupled in the sub-scanning direction as a long line image in the main scanning on the deflection reflection surface of the polygon mirror 214-1 of the polygon scanner 214 by the action of the cylindrical lens 214 right. Imaged.
The semiconductor laser 211 (right and left), the coupling lens 212 (right and left), and the cylindrical lens 214 (right and left) constitute the light source means 210. The light source means 210 includes two ribs 205 right and 205 left. Deployed in the area between.

The right side of the semiconductor laser 211 emits two laser beams (linearly polarized light) having polarization planes orthogonal to each other, and is respectively S-polarized light and P-polarized light with respect to the deflecting reflection surface of the polygon mirror 214-1. A rotation angle around the optical axis is set. The emitted beams from the two semiconductor lasers that emit linearly polarized laser beams may be combined so as to be respectively S-polarized light and P-polarized light with respect to the deflecting reflection surface of the polygon mirror.
Further, the number of S-polarized and P-polarized laser beams is not limited to one, but may be plural.
As shown in FIG. 4, the laser beams 219C and 219K reflected from the deflection reflection surface of the polygon mirror 214-1 pass through the right side of the opening 207 formed on the right side of the rib 205 via the right side of the scanning optical system 215, Incident to the right of the light beam separating means 217.
The light beam separation means 217 has a function of reflecting the laser beam 219C and transmitting the laser beam 219K according to the polarization characteristics of the laser beams 219C and 219K. The two laser beams 219C and 219K, whose optical paths are separated by the light beam separation means 217, are scanned by the folding mirrors 218C, 218K1, and 218K2, respectively, and then scanned on the photosensitive drums 9C and 9K as beam spots. .

The scanning optical systems 215 right and 215 left cover in order not to interfere with the laser beams 219C and 219M reflected from the light beam separating means 217 right and 217 left, and to be less susceptible to thermal deformation of the optical housing 101. Mounted on member 101-1.
By assembling the cover member 101-1 to the optical housing using a stepped screw or the like, it is possible to reduce the influence of thermal deformation of the optical housing. In this case, in order to prevent dust from entering the inside of the optical scanning device (optical housing 101), a dustproof member (not shown) made of foamed urethane or the like is attached between the optical housing 101 and the cover member 101-1. Good.
As shown in FIG. 3, a laser beam passage window 101-2 is formed in the cover member 101-1 corresponding to each photosensitive drum. Each passage window 101-2 is provided with a dustproof glass (not shown).

As shown in FIG. 6, the polygon scanner 214 has a configuration in which a polygon mirror 214-1 is mounted on a base member (PCB substrate) 224. The polygon mirror 214-1 is sealed in a state surrounded by the side wall 223, the cover member 100-2, and the three parallel flat glass plates 222a, 222b, and 222c. Thereby, it is possible to prevent the generation of airflow (forced convection) due to the rotation of the polygon mirror 214-1, the heat transfer of the heat generated by the driving of the polygon scanner 214, the heat conduction inside the member of the optical housing 101, and Heat transfer by natural convection is dominant.
In particular, when the optical housing 101 is made of a material having a high thermal conductivity such as a metal, the influence of the thermal conduction is dominant.

  In the present embodiment, as shown in FIG. 2, the folding mirrors 218C, 218K1, and 218K2 are supported by inserting end portions into substantially rectangular openings formed in the front side wall 202 and the rear side wall 201 and bridging them. However, it may be configured to be supported by a support portion formed on the bottom plate 206. Similarly, the scanning optical systems 215 right and 215 left may not be mounted on the cover member 101-1, but may be mounted on a stay member supported across the front side wall 202 and the rear side wall 201. .

As shown in FIG. 4, the right beam separation means 217 reflects the laser beam 219C and functions as a “folding mirror” for the laser beam 219C. Therefore, in the present embodiment, the number of folding mirrors in all stations is an even number (two).
In this way, by adopting a configuration in which the optical path is separated using the optical path separating means, it is possible to make the optical housing (optical scanning device) thinner, that is, to reduce the dimension in the Z-axis direction.
Here, the “station” indicates an optical path from the light source (semiconductor laser) to the photosensitive drum, optical elements, components of the optical housing that supports them, and the like, and exists for each development color.

As a comparison, an example in which the optical path separation means is not used is shown in FIG. Since the polygon mirror has a two-stage configuration, the thickness of the optical housing (dimension in the Z-axis direction) is increased by the distance between the two polygon mirrors (distance in the Z-axis direction).
As shown in FIG. 4, the laser beams 219M and 219Y emitted from the left of the semiconductor laser 211 also scan on the photosensitive drums 9M and 9Y as beam spots in the same manner as described above.

Hereinafter, thermal deformation of the optical housing will be described.
[Deformation mode of thermal deformation of optical housing]
A simulation using the finite element method was performed on the simulation model shown in FIG. 8 to calculate the thermal deformation of the optical housing when the polygon scanner was rotated.
An example of the result is shown in FIG. 9 and FIG. This is an example in the case where the height H of the rib is 20 mm (20 mm / 30 mm = 2/3 when normalized with the height of the optical housing being 30 mm). In this simulation, unnecessary shapes for the simulation, that is, shapes that do not affect the calculation results (holes, etc.) were deleted.
The material of the optical housing was made of industrially general-purpose aluminum. As a heat source, a “sealed type” polygon scanner shown in FIG. 6 is assumed, and a heat transfer coefficient assuming heat radiation by natural convection is set between the optical housing surface and the atmosphere. As a heat generation condition of the polygon scanner, a temperature of 60 ° C. was set at the center of the optical housing. The ambient temperature was 32 ° C. The four corners of the bottom of the optical housing were restrained.

As shown in FIG. 9, the temperature distribution has a substantially concentric shape centered on the heat source.
As shown in FIG. 10, the thermal deformation showed a deformation mode in which the XZ section had a positive (plus) curvature and the YZ section had a negative (minus) curvature. See FIG. 14 for the sign of curvature.

Variations in the scanning line position (sub-scanning beam spot position) accompanying thermal deformation of the optical housing will be described.
[When optical housing shows deformation mode with positive curvature in XZ section]
When the optical housing shows a deformation mode having a positive curvature in the XZ section, as can be seen from the contour lines of the displacement in the Z-axis direction shown in FIG. 10, the rear side wall 201 and the front side wall 202 that support the folding mirrors 219K1 and 219K2 etc. Is deformed in the same direction as the deformation mode in the XZ cross section.
Therefore, as shown in FIG. 11, the scanning lines on the photosensitive drums 9Y and 9M move in the + X direction (rightward), and the scanning lines on the photosensitive drums 9C and 9K move in the −X direction (leftward). .

[When optical housing shows deformation mode with negative curvature in YZ section]
The case where the optical housing shows a deformation mode in which the curvature is negative in the YZ section will be described with reference to the schematic diagram of FIG.
FIG. 12A is an optical path diagram in which the optical path is expanded and displayed by removing the folding mirrors 218K1 and 218K2. Since the optical housing shows a deformation mode in which the XZ cross section has a positive curvature, the rear side wall 201 to which the right side of the semiconductor laser 211 is fixed is deformed so that the upper end side is inclined to the + Y side. As a result, the optical axis emitted from the light source means (semiconductor laser, coupling lens, cylindrical lens) changes upward in the sub-scanning section (here, the YZ section).
In this case, the scanning line position on the photosensitive drum 9K moves downward as indicated by an arrow in the drawing.

FIG. 12B is an optical path diagram corresponding to the layout of the present embodiment shown in FIGS. The optical path of the laser beam is folded by the two (even number) folding mirrors 218K1 and 218K2, and the scanning line position on the photosensitive drum 9K moves to the right (+ X direction) as indicated by the arrow in the figure.
In other words, the effect when the optical housing shows a deformation mode with a positive curvature in the XZ section, that is, the effect of moving the scanning line position in the -X axis direction, and the optical housing has a negative curvature in the YZ section. The effect when the deformation mode is shown, that is, the effect of moving the scanning line position in the + X-axis direction cancel each other, and the scanning line position fluctuation can be reduced.

In the configurations described in Patent Documents 2 and 3, since materials having different coefficients of thermal expansion are combined, the configuration of the optical housing becomes complicated, and cost increases cannot be avoided.
On the other hand, by adopting the configuration of the present embodiment, it is possible to reduce the scanning line position fluctuation due to the influence of only the thermal deformation of the optical housing made of a single material without combining a plurality of materials. Is possible.
As shown in FIG. 12C, when the number of folding mirrors is an odd number, the scanning line position on the photosensitive drum 9K moves to the left (−X axis direction) as shown by the arrow in the figure. . Therefore, both effects are added, and the scanning line position fluctuation becomes large, which is not desirable.

The relationship between the height H of the rib and the thermal deformation of the optical housing will be described.
The deformation amount of the optical housing when the height H of the rib 205 right and 205 left in the optical housing 101 shown in FIG. 8 was changed was calculated. Here, the heights of the ribs 205 right and 205 left are constant values in the Y-axis direction. The result is shown in FIG.
The horizontal axis (rib height H) was standardized by the height (30 mm) of the side wall of the optical housing. The deformation of the optical housing and the sign of the curvature were defined as shown in FIG.
The legend “light source side” in FIG. 13A represents the deformation amount in the YZ section, and “scanning optical system side” represents the deformation amount in the XZ section. It can be seen that by regulating the rib height H, the “thermal deformation mode” of the optical housing can be regulated.
Thus, a member for restricting the thermal deformation mode of the optical housing (including the concept of adjustment and control) will be referred to as a “thermal deformation mode restricting member”.

  As in the optical housing 101 shown in FIG. 8, the ratio of the dimension in the X-axis direction (m1 = 400 mm) to the dimension in the Y-axis direction (m2 = 320 mm) is 400/320 = 1.25> 1, By forming the two ribs 205 right and 205 left parallel to the YZ section so as to connect the front side wall 202, the deformation mode shown in FIG. 10 can be obtained regardless of the height H of the ribs. In FIG. 8, the dimension m3 in the Z-axis direction is 34 mm.

The influence on the scanning line position will be described.
FIG. 13B shows the influence of the height H of the rib 205 right and 205 left on the scanning line position fluctuation when the sensitivity is about R1 = 1.2 and R2 = 2.0 in the K station. “Sensitivity” will be described later.
“Entire system” in the figure represents actual scanning line position fluctuation, “Light source side” represents contribution (component) due to eccentricity of light source means to scanning line position fluctuation, and “Scanning optical system” represents scanning line position fluctuation. It represents the contribution (component) due to the eccentricity of the scanning optical system.
By setting the height H of the rib within a range of 10 mm to 20 mm (standardized: 1/3 to 2/3 with respect to the height of the side wall), the scanning line position fluctuation can be suppressed to about 12 μm or less. .

The effect of “sensitivity” on scanning line position fluctuation will be described.
The deformation amount in the YZ section of the optical housing is N1, the variation amount of the scanning line position (sub-scanning beam spot position) on the photosensitive drum due to the deformation amount N1 is S1, and the ratio between the variation amount S1 and the deformation amount N1 is The sensitivity is R1.
Further, the deformation amount in the XZ section of the optical housing is N2, the variation amount of the scanning line position on the photosensitive drum due to the deformation amount N2 is S2, and the ratio of the variation amount S2 and the deformation amount N1 is sensitivity R2.
Sensitivity R1 and R2 are both positive numbers. The deformation amounts N1 and N2 are as shown in FIG.

The sensitivities R1 and R2 differ depending on the specifications (magnification, focal length, etc.) of the optical system and scanning optical system of the light source means and the optical path layout. That is, for example, when the sensitivity R1 is very large, the fluctuation amount S1 tends to be large even if the deformation amount N1 is relatively small.
Therefore, in order to cancel the fluctuation amounts S1 and S2, it is necessary to appropriately set the deformation amounts N1 and N2 according to the sensitivities R1 and R2.
At this time, by appropriately setting the deformation amounts N1 and N2 so that at least “N1 × N2 <0”, the above “scanning line position (sub-scanning beam spot position) variation due to thermal deformation of the optical housing” As shown in FIG. 6, since the contribution by the pre-polygon optical system and the contribution by the post-polygon optical system in the beam traveling direction are canceled with each other, the generation amount of the scanning line position variation can be reduced. In other words, the scanning line position fluctuation due to the eccentricity (optical axis deviation) of the light source means can be canceled by the eccentricity of the folding mirror (and the scanning optical system).
On the other hand, in the case of “N1 × N2> 0”, the amount of occurrence of the scanning line position fluctuation becomes large.
Further, it is more desirable that the absolute value of R1 · N1 and the absolute value of R2 · N2 be substantially equal, because the amount of occurrence of the scanning line position fluctuation can be made substantially zero.

The optimization of the rib height H will be described.
As described with reference to FIG. 13A in “Relationship Between Rib Height H and Thermal Deformation of Optical Housing”, the deformation amount of the optical housing (ie, the eccentric amounts N1 and N2) depends on the rib height H. Can be set.
As described above, by optimizing the height H of the rib, it is possible to reduce the variation in the scanning line position on the photosensitive drum.
As shown in FIG. 13A, the deformation amount of the optical housing is the minimum value when the standardized rib height H is 1/3 (= 0.333). In this respect, it is possible to optimize the reduction of the scanning line position fluctuation regardless of whether the rib height H is set to a value of 1/3 or less or to a value of 1/3 or more. .
On the other hand, the vibration characteristics of the optical housing also change depending on the height H of the rib. The influence of the rib height H on the resonance frequency was calculated using modal analysis. The result is shown in FIG.
As the height H of the rib is increased, the primary resonance frequency tends to increase.
Therefore, the standardized rib height should be set to 1/3 or higher in order to achieve both the reduction in scanning line position variation due to thermal deformation of the optical housing and the optimization of vibration characteristics (primary resonance frequency). Is desirable.
In the present embodiment, the ribs 205 right and 205 are thermal deformation mode restricting members, and the restriction by the rib 205 right and 205 is the height of the rib 205 right and 205 in the Z-axis direction with a predetermined dimension (1 / of the side wall height). 3 to 2/3).

A second embodiment will be described with reference to FIGS. 16 and 17.
Note that the same parts as those in the above embodiment are denoted by the same reference numerals, and unless otherwise specified, description of the configuration and functions already described is omitted, and only the main part will be described (the same applies to other embodiments below).
As shown in FIG. 16, the rib in the present embodiment has a shape whose height decreases from the end portion in the main scanning direction toward the center portion. In other words, the triangle shape is removed from the upper part in the Z-axis direction.
The heights of the right and left ribs 205 at the positions connected to the rear side wall 201 and the front side wall 202 are the same as the rear side wall 201 and the front side wall 202 (D0 = 30 mm), but the heights are appropriately changed. It doesn't matter. The width W of the rib was 4 mm.
The upper part in the Z-axis direction of the ribs 205 right and 205 left is cut in a triangular shape so that the height changes linearly from the position connected to the rear side wall 201 and the front side wall 202 to the center part.

The amount of deformation of the optical housing when the triangular height (depth of the cut shape) D at the center was changed was calculated. The result is shown in FIG. The horizontal axis (depth D) was normalized by the height of the side wall of the optical housing (D0 = 30 mm). It can be seen that the “thermal deformation mode” of the optical housing can be regulated by adjusting the depth D of the cut shape.
That is, the restriction by the rib 205 right and 205 as the thermal deformation mode restriction member in the present embodiment is to set D to a predetermined dimension.
FIG. 17B shows the influence of the cut shape depth D on the scanning line position fluctuation when the sensitivity is about R1 = 1.9 and R2 = 2.7 in the K station.
In the case of the present embodiment, by setting D / D0 = 0.44-0.89, that is, D = 13.2 mm-26.7 mm, the scanning line position variation can be suppressed to about ± 10 μm or less. More preferably, by setting D / D0 = 0.56 (D = 16.8 mm) of the rib, the scanning line position variation can be made substantially 0 μm.
In the present embodiment, the rib 205 is cut so that it changes linearly from the right and 205 left end portions (connecting portions with the rear side wall 201 and the front side wall 202) to the center portion, but as a shape cut with a curve. It doesn't matter.

A third embodiment will be described based on FIGS. 18 and 19.
In the present embodiment, the ribs 205 right and 205 left are set to the same height as the rear side wall 201 and the front side wall 202 (D0 = 30 mm) at the center, and as they move toward the rear side wall 201 and the front side wall 202. The shape is such that the height decreases linearly.
A difference (depth) between the heights of the right and left ribs 205 and the height of the rear side wall 201 and the front side wall 202 at a portion connected to the rear side wall 201 and the front side wall 202 was defined as E. The width W of the rib was 4 mm.

The amount of deformation of the optical housing when E was changed was calculated. The result is shown in FIG.
The horizontal axis (depth E) was normalized by the height of the side wall of the optical housing (D0 = 30 mm). It can be seen that the “thermal deformation mode” of the optical housing can be regulated by adjusting the depth E of the cut shape.
That is, the restriction by the rib 205 right and 205 as the thermal deformation mode restriction member in the present embodiment is to set E to a predetermined dimension.
FIG. 19B shows the influence of the depth E of the cut shape on the scanning line position fluctuation when the sensitivity is about R1 = 4.0 and R2 = 3.0 in the K station.
In the case of the present embodiment, by setting E / D0 = 0.37 to 0.72 (that is, E = 11.1 mm to 21.6 mm), the scanning line position fluctuation can be suppressed to about ± 10 μm or less.
As described above, in the second embodiment, the scanning line position fluctuation can be suppressed to about ± 10 μm or less in a wider range (D / D0 = 0.44 to 0.89) than in the present embodiment, and the second embodiment is better. It can be said that this is a more preferable configuration.

Based on FIG.5 and FIG.20, 4th Embodiment is described.
This embodiment is characterized in that the width (thickness) of the rib in the X-axis direction is defined.
The deformation amount of the optical housing when the width W of the rib 205 right and 205 left in the optical housing 101 shown in FIG. 5 was changed was calculated. In the present embodiment, the height of the rib 205 right and 205 left is H = 20 mm. The rib 205 right and 205 left connect the rear side wall 201 and the front side wall 202.
The result is shown in FIG. It can be seen that the “thermal deformation mode” of the optical housing can be regulated by adjusting the rib width W. That is, in the present embodiment, the restriction by the ribs 205 right and 205 as the thermal deformation mode restricting member is to set the width W to a predetermined dimension.

FIG. 20B shows the effect of the width W of the rib 205 right and 205 left on the scanning line position fluctuation in the K station when the sensitivity R1 = about 4.0 and R2 = about 4.9.
In the case of this embodiment, setting the rib width W to 2.0 mm or more can stabilize the scanning line position variation to about 3 to 10 μm, which is a desirable configuration.

The fifth embodiment will be described based on FIGS. 21 and 22.
In this embodiment, the number of ribs is restricted by setting to a predetermined number.
An optical housing 101 having a plurality of pairs of ribs is shown in FIG. FIG. 21 shows an example of an optical housing 101 having three pairs of ribs. The height H of the rib was 20 mm, and the width W was 3 mm.
The rib 205 right and 205 left connect the rear side wall 201 and the front side wall 202. Thus, the deformation amount of the optical housing when the number M of ribs was changed was calculated. The result is shown in FIG.
It can be seen that the “thermal deformation mode” of the optical housing can be regulated by adjusting the number M of ribs arranged.
That is, in the present embodiment, the restriction by the ribs 205 right and 205 as the thermal deformation mode restriction member is to set the number of ribs arranged to a predetermined number.

FIG. 22B shows the influence of the arrangement number M of the rib 205 right and 205 left on the scanning line position fluctuation in the case where the sensitivity is about R1 = 3.0 and R2 = 2.6 in the K station.
In the case of the present embodiment, by setting the number of ribs to be M = 1 to 3 pairs, the scanning line position variation can be suppressed to about 18 μm or less. More desirably, by setting the number of ribs to be M = 2 pairs, the scanning line position variation can be made substantially 0 μm.

DESCRIPTION OF SYMBOLS 9 Photosensitive drum as a to-be-scanned surface 101 Optical housing as a housing | casing 201, 202, 203, 204 Side wall 205 Rib as a thermal deformation mode control member 206 Bottom plate 210 Light source means 214 Polygon scanner as a deflection means 215 Scanning optical system 218 Folding mirror

JP 2010-39149 A Japanese Patent Laid-Open No. 9-2225777 JP 2006-201626 A JP 2005-181714 A Japanese Patent No. 4299103 Japanese Patent No. 4634819

Claims (8)

A light source means, a deflecting means for deflecting the laser beam emitted from the light source means, and a scanning optical system for forming an image of the laser beam deflected by the deflecting means on the surface to be scanned as a light spot,
When the axis direction of the rotation axis of the deflecting means is the Z-axis direction as the sub-scanning direction, the main scanning direction is the Y-axis direction, and the direction orthogonal to the Z-axis direction and the Y-axis direction is the X-axis direction,
The scanning optical system is provided on both the + X-axis direction side and the −X-axis direction side across the YZ cross section including the Y-axis direction and the Z-axis direction, and the light source means includes the + X-axis with respect to the X-axis direction. Arranged between the scanning optical system on the direction side and the -X axis direction side,
An even number of folding mirrors for folding the optical path of the laser beam are provided between the deflecting means and the surface to be scanned,
A housing for housing the light source means, the deflecting means, the scanning optical system, and the folding mirror;
The housing is composed of a substantially rectangular bottom plate parallel to the XY plane including the X-axis direction and the Y-axis direction, and side walls formed to extend in the Z-axis direction on the four sides of the bottom plate.
In the optical scanning device, the dimension in the X-axis direction of the housing is larger than the dimension in the Y-axis direction.
The casing has ribs that connect the side wall on the + Y axis direction side and the side wall on the −Y axis direction side of the casing on the + X axis direction side and the −X axis direction side, respectively, across the YZ section. Prepared,
The thermal deformation mode in the YZ cross section of the housing accompanying the driving of the deflection means and the deformation mode in the XZ cross section including the X axis direction and the Z axis direction are opposite to each other in terms of the sign of curvature. An optical scanning device having a thermal deformation mode restricting member for restricting to the above. The optical scanning device according to claim 1,
The optical scanning device according to claim 1, wherein the thermal deformation mode regulating member is the rib, and the rib is regulated by setting a height of the rib to a predetermined dimension. The optical scanning device according to claim 2,
The rib is shaped so as to be higher in the vicinity of the side wall on the + Y axis direction side and on the side wall on the −Y axis direction side and lower in the center part in the Y axis direction. An optical scanning device characterized in that the height is regulated by setting the height at a predetermined dimension. The optical scanning device according to claim 1,
An optical scanning device characterized in that the thermal deformation mode restricting member is the rib, and the thickness, which is the width of the rib in the X-axis direction, is set to a predetermined dimension. The optical scanning device according to claim 1,
The optical scanning device according to claim 1, wherein the thermal deformation mode regulating member is the rib, and the number of the ribs arranged is regulated to a predetermined number. The optical scanning device according to claim 1,
The deformation amount in the YZ cross section of the housing due to the driving of the deflection means is N1, and the variation amount of the beam spot position in the sub-scanning direction on the scanned surface due to the deformation amount N1 is S1, and the variation amount S1 and the deformation amount The ratio to the quantity N1 is the sensitivity R1,
The deformation amount in the XZ cross section of the casing due to the driving of the deflection means is N2, and the variation amount of the beam spot position in the sub-scanning direction on the scanning surface due to the deformation amount N2 is S2, and the variation amount S2 and the deformation amount When the ratio to the quantity N2 is the sensitivity R2,
An optical scanning device characterized in that regulation is performed by setting the shape or number of the ribs so that the absolute value of R1 · N1 and the absolute value of R2 · N2 are substantially equal. In the optical scanning device according to any one of claims 1 to 6,
At least one of the folding mirrors is an optical path separating unit that reflects at least one of a plurality of incident laser beams and transmits the remaining laser beams. An electrostatic latent image is formed on the image carrier by an optical scanning device based on the image information, the electrostatic latent image is visualized by a developing device, and the visible image is transferred to a recording medium indirectly or directly. In an image forming apparatus that forms an image,
The image forming apparatus according to claim 1, wherein the optical scanning device is the one according to claim 1.

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