Abstract:
Provided is: a resin molded article for an optical element wherein high surface precision is kept since an abnormal appearance-formed portion such as a hesitation mark is effectively formed outside an optical surface without cutting off the portion and an optical surface itself can be less likely to be influenced by shrinkage with hardening such as sink; a method and device for manufacturing the same; and a scanning optical device. The resin molded article for an optical element which comprises first surface portion at a part of the surface of a resin molded base and comprises a hollow portion found by injecting a fluid into the inside of the base from the outside. Assuming that the distance between the first end of the base and an end of the first surface portion, the end being close to the first end, is A and the distance between the second end of the base, the end being other than the first end and being on the opposite side across the first surface portion, and the end of the first surface portion, the end being on the side close to the second end, is B, the relations of (A&gt;0, B&gt;0, A≦B) are satisfied.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to a resin molded article for optical element, a method for manufacturing a resin molded article for optical element, a device for manufacturing a resin molded article for optical element, and a scanning optical device; particularly to a resin molded article for optical element wherein a hollow portion is formed by injecting a fluid into the resin having been charged into the cavity of a mold; a method for manufacturing resin molded article for optical element; a device for manufacturing a resin molded article for optical element; and a scanning optical device. 
       BACKGROUND ART 
       [0002]    The aforementioned optical element made of glass, metal or ceramics is widely known. In recent years, a resin-made optical element has come to be employed to ensure molding ease, greater freedom of designing, and reduced costs. 
         [0003]    The aforementioned optical element has been employed in a great variety of fields. One of the commonly known examples of application is found in such a device as an optical information recording/reproduction device and optical scanning device wherein the light emitted from a light source is converged and an image is formed on a recording surface and others so that recording and reproduction are performed. However, these devices have been requested to provide higher image quality and higher definition, hence, a higher recording density in recent years. However, to achieve higher definition, each component used is required to provide a high degree of control precision. Since the optical element as one of the constituting element allows passage and reflection of the light emitted from the light source, and converges, deflects and deforms the light, the optical surface of the optical element is required to provide a high degree of surface precision. In recent years, attention has been drawn to the short-wave blue laser ensuring a longer service life and stable output. Since this laser ensures easy formation of a still smaller spot, the optical element must have a high degree of surface precision capable of meeting such a sophisticated function. 
         [0004]    However, amid the requirements for a higher degree of surface precision, big technological problems unnoticed heretofore have come to the surface. The most prominent problem is related to an impact on deformation of the optical surface due to the warping and sink marks caused by shrinkage at the time of resin hardening in the process of resin injection molding. Especially in the optical element provided with fθ characteristic, the impact of the warping occurring in the scanning direction has come to the surface. Thus, the conventional injection molding fails to ensure the quality of an optical component characterized by such high precision. Further, as described above, when the optical element is to be applied to the short wave laser beam, for example, blue laser, the weatherability of the resin lens presents a further problem in ensuring high surface precision. 
         [0005]    To solve this problem, the inventors of the present invention paid attention to the effect of hollow injection molding, and have studied the possibility of application to the optical component. If the hollow injection molding technique is used to perform hollow injection molding, the tensile stress due to shrinkage at the time of hardening that causes the warping and sink mark of the molded product will be released in a hollow portion. When the tensile stress takes the form of a sink mark on the surface of the hollow portion, the warping and sink mark appearing on the surface of the molded product can be mitigated. 
         [0006]    In one of the methods of creating a hollow portion in a resin molded article, a mold is charged with a molten resin by injection. Then the mold is filled with a compressed gas as a fluid by the injection nozzle or the gas filling nozzle provided in the mold cavity. However, the flow speed at the leading edge of the molten resin may be changed by a time lag in the step of filling with gas subsequent to resin charging, or the flow is suspended, in some cases. This will result in such a defect of unsightly appearance as a hesitation mark on the leading edge of the molten resin, and will cause serious deterioration of the surface precision. 
         [0007]    To solve this problem, in one of the conventional techniques (e.g., Patent Literature 1), a mold is charged with resin from the injection nozzle. When the mold is fully charged, the mold is filled with a compressed gas from a different gate. In this case, the excess resin is fed to a flowing resin receiver through a resin outflow tract. A detecting device is used to detect that the resin has reached a prescribed position, before the gas reaches the resin outflow tract. Then a switching device is used to close the resin outflow tract, and the resin is solidified under pressure, whereby the molded product is produced. After that, the resin outflow tract and flowing resin receiver are cut off by the switching member in the mold. It is demonstrated that no defect of unsightly appearance such as a hesitation mark appears. 
       BACKGROUND ART DOCUMENT 
       [0008]    Japanese Unexamined Patent Application Publication No. Hei 11 (1999)-138577 
       SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
       [0009]    However, according to this conventional method, a resin outflow tract and flowing resin receiver as unwanted portions for a resin molded article are provided. A compressed gas is filled after the resin has reached the outflow tract. The portion containing a defect of unsightly appearance such as a hesitation mark is formed on the unwanted molding portion outside the position to be cut. After that, the unwanted molding portion is cut off at the position to be cut. It has been shown, however, such an unwanted cutting operation subsequent to molding is not applicable to the scanning optical element equipped, on the periphery of the cut portion, with an optical surface required to provide a high degree of surface precision, especially to an optical element wherein high-density recording and reproduction is performed using a short-wave light. At the same time, an optical element involves a technological problem that must be solved together with the problem of hesitation marks. 
         [0010]    In the optical element, the optical surface formed on part of the substrate requires a complete solution of the aforementioned problem caused by the shrinkage at the time of resin hardening. This requires the region formed on the hollow portion to be controlled below this optical surface to some extent. This is performed by filling the cavity with a fluid while the hollow portion is in the process of being molded. This makes it necessary to anticipate the region filled with the charged resin by the fluid. However, in the case of an optical component, differently from other molded products, the surface precision is affected also by the resin charged position and fluid inflow position, and this imposes restrictions. Thus, the resin charged position and fluid inflow position are preferably designed in such a way that the resin is emitted from one end of the cavity outside the region where the optical surface is formed. 
         [0011]    This requires the profile of the optical component to be designed to ensure that, even when the resin charging and fluid filling operations are performed from such a restricted position, a hollow region is formed below the optical surface to some extent, and a defect of unsightly appearance such as a hesitation mark will not adversely affect the optical surface. 
         [0012]    In view of the problems described above, it is an object of the present invention to provide a resin molded article for an optical element capable of providing an effective solution to problems involved in the surface precision deteriorated by sink marks resulting from shrinkage at the time of resin hardening, and a defect of unsightly appearance such as a hesitation mark, a method and device for manufacturing this resin molded article, and a scanning optical device. 
       Means for Solving the Problems 
       [0013]    To solve the aforementioned problems, a first embodiment of the present invention is a resin molded article for an optical element including: a first surface portion provided on part of the surface of the substrate formed of resin; and a hollow portion fanned by filling the substrate interior with a fluid from outside; wherein, assuming that the distance between the first end of the substrate and the end of the first surface portion close to this first end is “A”, and the distance between the second end on the opposite side through the first surface portion, the second end being the end different from the first end of the substrate, and the end of the first surface portion close to the second end is “B”, the following relationship is satisfied: 
         [0000]      A&gt;0 
         [0000]      B&gt;0 
         [0000]      A≦B
 
         [0014]    A second embodiment of the present invention is the resin molded article for optical element described in the aforementioned first embodiment, wherein the surface roughness Ra of the entire first surface portion satisfies Ra≦5 (nm). 
         [0015]    A third embodiment of the present invention is the resin molded article for optical element described in the aforementioned first embodiment, wherein a mirror portion is formed on the first surface portion. 
         [0016]    A fourth embodiment of the present invention is a scanning optical device including: a light source; a deflection means for deflecting the outgoing light emitted from this light source; a converging means wherein the light emitted from this light source enters and converges onto the deflection means; and an image forming optical system wherein the image of the light deflected by the deflection means is formed on the scanned surface; wherein at least one of the optical elements constituting the image forming optical system has one surface portion on part of the surface of a long substrate formed of resin, and a hollow portion formed by injecting a fluid into the substrate from the outside; wherein assuming that the distance between the first end of the substrate and the end of the first surface portion close to this first end is “A”, and the distance between the second end on the opposite side through the first surface portion, the second end being the end different from the first end of the substrate, and the end of the first surface portion close to the second end is “B”, the following relationship is satisfied: 
         [0000]      A&gt;0 
         [0000]      B&gt;0 
         [0000]      A≦B
 
         [0017]    A fifth embodiment of the present invention is the scanning optical device described in the aforementioned fourth embodiment, wherein the surface roughness Ra of the entire first surface portion satisfies Ra≦5 (nm). 
         [0018]    A sixth embodiment of the present invention is the scanning optical device described in the aforementioned fourth or fifth embodiment, wherein the first surface portion is provided with a mirror surface section for reflecting the outgoing light. 
         [0019]    A seventh embodiment of the present invention is the scanning optical device described in the aforementioned sixth embodiment, wherein the surface roughness Ra of the entire first surface portion satisfies Ra≦5 (nm). 
         [0020]    An eighth embodiment of the present invention is a method for manufacturing a resin molded article for an optical element wherein, in a resin molded article for an optical element having a first surface portion on part of the surface of the substrate formed of resin, and a hollow portion formed by injecting a fluid into the substrate from the outside, assuming that the distance between the first end of the substrate and the end of the first surface portion close to this first end is “A”, and the distance between the second end on the opposite side through the first surface portion, the second end being the end different from the first end of the substrate, and the end of the first surface portion close to the second end is “B”, the following relationship is satisfied: 
         [0000]      A&gt;0 
         [0000]      B&gt;0 
         [0000]      A≦B
 
         [0021]    wherein the aforementioned method for manufacturing a resin molded article for an optical element includes: a step of preparing a first mold having a transfer surface for transferring the first surface portion; and a second mold provided opposed to the first mold to form a cavity by clamping the mold jointly with the first mold; a step of an injection step for injecting a molten resin from one of the cavity ends into the cavity, a detection step for detecting that the leading edge of the resin charged in the injection step is located at a prescribed position; and a fluid injection step for controlling the charging with resin based on the detection step and to inject a fluid into the cavity to form a hollow portion inside the cavity. 
         [0022]    A ninth embodiment of the present invention is the method for manufacturing a resin molded article for an optical element described in the aforementioned eighth embodiment wherein the surface roughness Ra of the entire first surface portion satisfies Ra≦5 (nm). 
         [0023]    A tenth embodiment of the present invention is the method for manufacturing a resin molded article for an optical element described in the aforementioned eighth or ninth embodiment, further including a mirror surface section forming step for forming a minor surface section on the first surface portion of the resin molded article obtained subsequent to the fluid injection step. 
         [0024]    A eleventh embodiment of the present invention is the method for manufacturing a resin molded article for an optical element described in any one of the aforementioned eighth, ninth and tenth embodiments, wherein, in the fluid injection step, injection of fluid starts after the lapse of a prescribed time from suspension of charging with resin. 
         [0025]    A twelfth embodiment of the present invention is a device for manufacturing a resin molded article for an optical element wherein, in a resin molded article for the optical element having a first surface portion on part of the surface of a substrate formed of resin, and a hollow portion formed by injecting a fluid into the substrate from the outside, assuming that the distance between the first end of the substrate and the end of the first surface portion close to this first end is “A”, and the distance between the second end on the opposite side through the first surface portion, the second end being the end different from the first end of the substrate, and the end of the first surface portion close to the second end is “B”, the following relationship is satisfied: 
         [0000]      A&gt;0 
         [0000]      B&gt;0 
         [0000]      A≦B
 
         [0026]    the aforementioned device for manufacturing a resin molded article further including: a first mold having a transfer surface for transferring the first surface portion; a second mold provided opposed to the first mold to form a cavity by clamping the mold jointly with the first mold; a charging means for injecting a molten resin from one of the cavity ends into the cavity; a detection means for detecting that the resin charged into the cavity by the charging means is located at a prescribed position; and a fluid injection means for controlling charging with resin by the detection means and injection of a fluid into the cavity by the fluid injection means. 
       Advantages of the Invention 
       [0027]    The present invention provides a resin molded article for an optical element, a method for manufacturing such a resin molded article for the optical element, and a device for manufacturing such  a  resin molded article for the optical element, wherein the aforementioned resin molded article for the optical element is characterized by a high degree of surface precision because the molded portion with abnormal appearance such as a hesitation mark in the present invention can be effectively formed outside an optical surface without having to cut off and removing such a molded portion, and an optical surface itself can be made immune to shrinkage by hardening such as sink. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is an explanatory diagram showing a laser beam scanning optical device incorporating an optical element in a first embodiment of the present invention; 
           [0029]      FIG. 2  is a cross sectional view showing that the resin molded article for the optical element is cut in the direction of length; 
           [0030]      FIG. 3  is a plan view showing the resin molded article for the optical element; 
           [0031]      FIG. 4   a  is a cross sectional view of the mold when cut by a perpendicular line including a bisector in the direction of thickness, and  FIG. 4   b  is a cross sectional view of the mold when cut by a perpendicular line including a bisector in the direction of length; 
           [0032]      FIG. 5  is a functional block diagram showing an injection molding machine equipped with a detecting means; 
           [0033]      FIG. 6  is a time chart showing the relationship between the detection temperature and injection of compressed gas; 
           [0034]      FIG. 7  is a flow chart showing a step of manufacturing the resin molded article for the optical element; 
           [0035]      FIG. 8  is a functional block diagram showing the injection molding machine as a variation of the present invention; 
           [0036]      FIG. 9  is a time chart showing the relationship between the detection temperature and injection of compressed gas; 
           [0037]      FIG. 10  is a flow chart showing the step of manufacturing a resin molded article for the optical element as an variation of the present invention; 
           [0038]      FIG. 11  is a functional block diagram showing the injection molding machine as another variation of the present invention; 
           [0039]      FIG. 12  is a flow chart showing a step of manufacturing the resin molded article for the optical element as still another variation of the present invention; 
           [0040]      FIG. 13  is a plan view showing the resin molded article for the optical element in a second embodiment of the present invention; and 
           [0041]      FIG. 14  is a cross sectional view showing the resin molded article for the optical element. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
       [0042]    (Structure) 
         [0043]    Referring to  FIG. 1 , the following describes the resin molded article for the optical element in a first embodiment of the present invention.  FIG. 1  is a diagram showing an example of a laser beam scanning optical device incorporating a resin molded article for the optical element. 
         [0044]    In  FIG. 1 , the laser beam scanning optical device includes a light source unit  1 , cylindrical mirror  2 , polygon mirror  3  as a deflection means, tonic lens  4 , plane mirrors  5  and  6 , and fθ mirror  10  with fθ characteristic. 
         [0045]    After having been converged into an approximately parallel beam by a collimating lens (not illustrated), the laser beam emitted from the light source unit  1  is reflected by the cylindrical mirror  2  and is converted into the form of an approximately straight line wherein the beam in the direction of length is parallel to the main scanning direction. Then the laser beam reaches the polygon mirror  3 . 
         [0046]    The polygon mirror  3  has four planes of polarization on the outer peripheral portion and is driven at a constant speed in the counterclockwise direction. The laser beam is deflected at a constant angular velocity in the main scanning direction by the rotation of the polygon mirror  3  and is led to the toric lens  4 . In this case, the toric lens  4  has different powers in the main scanning direction and in the sub-scanning direction, and the laser beam is converged on the scanned surface in the sub-scanning direction, whereby the deflected surface of the polygon mirror  3  and the scanned surface are kept in the relationship of conjugation. Thus, the planar inclination error of each deflecting surface of the polygon mirror  3  is corrected by combination with the cylindrical mirror  2 . 
         [0047]    The above description uses an example of a polygon mirror as a deflection means, without the present invention being restricted thereto. It goes without saying that a galvano mirror and other commonly known deflection means can be used so long as the incoming light is deflected in a different direction. 
         [0048]    The laser beam having passed through the toric lens  4  is reflected by the plane mirrors  5  and  6  and is further reflected by the fθ mirror  10 . After that, the laser beam is converged onto the photoreceptor drum  7 . The speed of the laser beam having been deflected at a constant angular velocity by the polygon mirror  3  is converted by the fθ mirror  10  to a constant linear velocity on the scanned surface (photoreceptor drum  7 ). The photoreceptor drum  7  is driven in the counterclockwise direction at a constant speed. An image is formed on the photoreceptor drum  7  by the main scanning operation of the laser beam by the polygon mirror  3 , rotation (sub-scanning) of the photoreceptor drum  7 , and modified laser beam output. 
         [0049]    As described above, the laser beam scanning optical device is made up of various types of optical elements. Especially, such substrates as the plane mirrors  5  and  6  and fθ mirror  10  are formed in a long tabular shape. A mirror surface is provided to reflect the laser beam received within a prescribed range in the direction of length, and an image is formed on the photoreceptor drum  7 . Thus, the structure is designed in such a way that the image quality is directly affected by the precision on the surface of the optical element provided with the mirror surface. 
         [0050]    Referring to  FIGS. 1 through 3 , the following describes the details of the structure of the resin molded article for the optical element.  FIG. 2  is a cross sectional view showing that the resin molded article for the optical element is cut in the direction of length.  FIG. 3  is a plan view showing the resin molded article for the optical element. 
         [0051]    The optical element is required to provide a high degree of mirror surface precision and dimensional precision, reduced weight, enhanced safety and durability, and economic viability. Such an optical element provides excellent production materials over wide-ranging fields including the materials for manufacturing electric and electronic components, automotive parts, medical goods, safety equipment, building materials and household appliances. 
         [0052]    As described above, the optical element of the present invention is exemplified by the plane mirrors  5  and  6  and fθ mirror  10  built in the laser printer. The substrates of the plane mirrors  5  and  6  and fθ mirror  10  built in the laser beam have hollow portions, and a hesitation mark is provided outside the surface characterized by a high degree of surface precision. The following describes the fθ mirror  10  as a typical example, and description of the plane mirrors  5  and  6  and other optical elements will be omitted. 
         [0053]    The fθ mirror  10  includes: a first surface portion  11  formed in a long tabular shape and having a prescribed range H 1  in the direction of length, provided with a mirror surface section  13  for reflecting the optical beam received within the prescribed range H 1 ; and a pair of second surface portions  12  arranged to sandwich the first surface portion  11  from the direction of length. The direction of length is defined as the lateral direction facing the sheet of  FIG. 2 , and the direction of thickness is defined as the vertical direction. The direction of width is defined as the longitudinal direction in  FIG. 3 . 
         [0054]    In the breadth of the direction of length, a prescribed range H 1  is kept within the region of the mirror surface section  13 , and the region of the mirror surface section  13  is kept within the region of the first surface portion  11 .  FIGS. 2 and 3  show the region of the mirror surface section  13  and that of the first surface portion  11  conforming to each other in the breadth of the direction of length. 
         [0055]    In the fθ mirror  10 , a long tabular substrate, a mirror surface section  13  located on one of the surfaces of the substrate and a third electrode  14  located inside the substrate on the back of the minor surface of the mirror surface section  13  are provided. Further, both ends of the hollow portion  14  are formed outside both ends of the mirror surface section  13  in the direction of length. This structure ensures that the tensile stress caused by shrinkage resulting from resin hardening is released into the hollow portion  14  having been formed. The warping in the direction of length caused by shrinkage resulting from resin hardening is mitigated over the entire mirror surface section  13 , with the result that the surface precision is enhanced. 
         [0056]    In the conventional technique, the mold is gripped by the molded article due to shrinkage resulting from resin hardening, and distortion of the mirror surface section  13  occurs due to resistance to mold release. In the present invention, however, the mirror surface section  13  is protruded from the substrate in the direction of thickness. This structure minimizes the distortion of the mirror surface section  13  due to resistance to mold release. Further, when the optical element (resin molded article) is manufactured, the mirror surface section  13  is corrected, for example, the thickness of the mirror surface section  13  is reduced by cutting partially or wholly. This correction may change the profile of the mirror surface section  13 . Even when the surface of the mirror surface section  13  is embedded into the substrate as a result of correction, the surface of the mirror surface section  13  can be kept protruded over the surface of the substrate after correction, by adjusting the length of the mirror surface section  13  protruded from the substrate in advance in anticipation of the correction of the mirror surface section  13 . 
         [0057]    In the resin molded article of the present embodiment, assume that the length of the mirror surface section  13  in the direction of length is L 1 , the length in the direction of width is W 1 , the length of the hollow portion  14  in the direction of length is L 2 , the length in the direction of width is W 2 , the length in the direction of thickness is D 2 , the length of the substrate in the direction of width is W 4 , and the distance from the end of the mirror surface section  13  to the end of the substrate with respect to one side in the direction of length is L 5 . It is preferred to design the structure wherein the distance L 3  from the end of the mirror surface section  13  to the end of the hollow portion  14  is 0≦L 3 &lt;L 5  with respect to one side in the direction of length. The distance W 3  from the end of the mirror surface section  13  to the end of the hollow portion  14  is 0≦W 3 &lt;W 2 /2 with respect to one side in the direction of width. 
         [0058]    Further, assume that A denotes the distance from the end of the first surface portion  11  to the end of the optical element on the same side, and B indicates the distance between the end on the first surface portion  11  formed on the opposite side through the hollow portion  14 , thus the end being different from the end of the first surface portion  11 , and the end of the optical element located on the same side. Under this condition, the optical element is required to have such a profile that meets the following relationship: A&gt;0 and B&gt;0. 
         [0059]    At the same time, when the resin and fluid are injected from the resin charging end J on the side A, in this case, A≦B (A≧B when resin and fluid are injected from side B) must be satisfied in order to ensure that the molded portion of unsightly appearance such as a hesitation mark is located outside the first surface portion  11 , and the hollow portion  14  is formed below the region corresponding to the first surface portion  11 . 
         [0060]    In the case of a smaller optical element, A and B are within the following range: 3.5≦A≦5.0 and 3.5≦B≦5.0. The aforementioned condition is more preferably satisfied. 
         [0061]    When D 1  is the length of the mirror surface section  13  protruding from the surface of the substrate in the direction of thickness, D 1  is within the range of 0.1 (mm)&lt;D 1 ≦3 (mm). When consideration is given to mold release, the lateral area of the mirror surface section  13  will be increased, and the resistance to mold release will also be increased. This will reduce the mirror surface precision on the periphery. To prevent this, it is preferred to meet 0.1 (mm)&lt;D 1 ≦0.3 (mm). 
         [0062]    The preferred relationship between the length W 1  of the mirror surface section  13  in the direction of width and the length W 2  of the hollow portion  14  is 0.01≦W 2 /W 1 ≦1. 
         [0063]    In  FIGS. 1 and 2 , the hollow portion  14  is arranged at the center both in the directions of width and thickness and is illustrated in a straight line in parallel with the mirror surface section  13 . This is only for the sake of schematic illustration, without imposing any restriction on the profile or positional relationship of the hollow portion  14 . 
         [0064]    A hesitation mark HM is formed on the second surface portions  12 . The hesitation mark HM can be formed at any position within the width of the second surface portions  12  in the direction of length. However, the hesitation mark HM is preferably provided as far away from the first surface portion  11  as possible. 
         [0065]    In the first embodiment, the fθ mirror  10  has been introduced as a resin molded article for the optical element molded in a long tabular form. However, it need not be a long one as long as it is a resin molded article for the optical element molded in a tabular form. A circular, elliptical or approximately square molded article can be used. In this case, the hollow portion  14  is provided along the first surface portion  11 . It is only required that the hollow portion  14  should be molded wider than the first surface portion in this direction. 
         [0066]    (Injection Molding Machine) 
         [0067]    The following describes the injection molding machine for manufacturing the substrate of the fθ minor  10  with reference to  FIGS. 1 through 6 .  FIG. 4   a  is a cross sectional view of the mold when cut by a perpendicular line including a bisector in the direction of thickness, and  FIG. 4   b  is a cross sectional view of the mold when cut by a perpendicular line including a bisector in the direction of length.  FIG. 5  is a functional block diagram showing an injection molding machine equipped with a detecting means  33 .  FIG. 6  is a time chart showing the relationship between the detection temperature and injection of compressed gas. 
         [0068]    The mold  42  having a cavity  31  has a charging means  32  for changing the cavity  31  with resin, a detecting means  33  for detecting the leading edge of the resin, a gas injection means  34  for injecting compressed gas, and a control means  35  for controlling the start and stop of the resin charging operation, and start and stop of the compressed gas injection 
         [0069]    (Mold) 
         [0070]    The cavity  31  has an internal surface for forming the first surface portion  11  and second surface portions  12  constituting the outer surface of the resin molded article for the optical element. Referring to  FIG. 4 , the following describes the profile of the mold.  FIG. 4   a  is a cross sectional view of the mold when cut by a perpendicular line including a bisector in the direction of thickness.  FIG. 4   b  is a cross sectional view of the mold when cut by a perpendicular line including a bisector in the direction of length between the internal surfaces of the cavity  31  including a first region  311  for forming the first surface portion  11  and a second region  312  for forming the second surface portions  12 . In  FIG. 4 , “A” indicates the distance between the cavity end on the resin charging side and fluid injection side, and the end of the first surface portion  11 ; and “B” denotes the distance between the other end and the end of the first surface portion  11 . 
         [0071]    Here, to achieve the surface precision used in the short-wave having a wavelength of 500 nm or less, the mirror surface forming section  315  is machined to a surface roughness Ra of 5 nm or less. This surface roughness Ra is preferably in the range of 2 to 3 nm. 
         [0072]    Referring to  FIG. 5 , the mechanism surrounding the mold in an injection molding machine will be described. A gate  321 , runner  322  and spool  323  are formed continuously on the cavity  31 . A heater (not illustrated) is provided along the cavity  31 , runner  322  and spool  323  (passage of the mold). This heater ensures that the molten resin having contacted the cavity  31  and passage of the mold will not be solidified by being cooled by thermal conduction and becoming less fluid. Instead of the heater, a temperature regulating water channel can be provided on the mold.  FIG. 5  shows the internal surface of the cavity  31  as the outside shape of the fθ mirror (resin molded article)  10 .  FIG. 5  also shows the gate  321 , runner  322  and spool  323  as an outside shape of the resin passing through them. 
         [0073]    (Charging Means) 
         [0074]    The charging means  32  is preferably mounted on the mold so that the resin will be charged from the direction of width of the fθ mirror  10  to the direction of length.  FIG. 5  shows the side of the fθ mirror  10  in the direction of width that denotes the far-right portion of the cavity  31 . 
         [0075]    The nozzle  324  of the charging means  32  communicates with the spool  323 . The charging means  32  has a screw (not illustrated) for extruding the molten resin from the nozzle  324 . The screw allows the molten resin to be fed from the nozzle  324  to the spool  323 , runner  322  and the gate  321  so that the cavity  31  is filled with resin. The distance traveled from the screw starting position or the time elapsed after start of screw traveling corresponds to the amount of the molten resin to be extruded (injection volume). The volumes of the mold passage from the spool  323  to the gate  321  and the cross sectional profile of the cavity  31  at each position in the direction of length are already known. This makes it possible to calculate the position of the leading edge of the molten resin charged into the cavity  31 , based on the distance traveled from the screw starting position or the time elapsed after the start of screw traveling. 
         [0076]    (Detecting Means(s)) 
         [0077]    The detecting means  33  is a temperature sensor for detecting the temperature on the internal surface of the cavity  31 . One or more detecting meanss  33  are arranged on the internal surface of the cavity  31  having the same range as that of the second region  312  in the direction of length, including the second region  312  of the internal surface of the cavity  31  for forming the second surface portions  12 . Here, the internal surface of the cavity  31  having the same range as that of the second region  312  in the direction of length refers to the internal surface of the cavity  31  provided in a circumferential shape in the same range as that of the second region  312  in the direction of length, and indicates the bottom surface  312  and double lateral wall surface  314 , when the second region  312  is assumed as a ceiling surface.  FIG. 5  indicates a detecting means  33  arranged on the bottom surface  313  opposed to the second region  312  (ceiling surface) on the side opposite the second region  312  on the gate side, with respect to the direction of length. The detecting means  33  is not restricted to a temperature sensor if it is a sensor capable of detecting the leading edge of the resin at the time of injection inside the cavity  31 . For example, an ultrasonic sensor or magnetic sensor can be used. 
         [0078]    The detecting means  33  can detect the leading edge of the resin having reached the second region  312  of the cavity  31 . The control means  35  receives the detected temperature t 1  from the detecting means  33  through the interface  38  as a detection signal. The control means  35  controls the charging means  32  and stops the resin charging operation, based on the detected temperature t 1  from the detecting means  33 . The control means  35  also controls the gas filling means  34   t o start the compressed gas injection. 
         [0079]    A detecting means  33  is provided on the internal surface of the cavity  31  having the same range as that of the second region  312  in the direction of length, including the second region  312 . This arrangement ensures that the surface precision of the first surface portion  11  is not adversely affected by the detecting means  33 . Further, the leading edge of the resin having reached the second region  312  is detected directly by the detecting means  33 , and the resin charging operation is stopped in response to this detection signal. This structure minimizes an error in time up to the start of the compressed gas injection operation subsequent to arrival of the leading edge of the resin to the second region  312  and suspension of the resin charging operation. This ensures the hesitation mark HM to be formed on the second surface portions  12 , and protects the surface precision of the first surface portion  11  against possible deterioration. 
         [0080]    (Gas Injecting Means) 
         [0081]    The gas filling means  34  includes a tank (not illustrated) for storing the compressed gas, a solenoid valve  341 , and an injection outlet  342  communicating with the cavity  31 . The control means  35  controls the open/close operation of the solenoid valve  341 . Any compressed gas can be used if it does not react or mix with the resin. For example, an inert gas can be used. When safety and economy are taken into account, nitrogen is preferably used because it is non-combustible and non-toxic, and does not require much cost. The injection outlet  342  is located on the bottom surface  313  in a region corresponding to the second region  312  of the internal surface of the cavity  31 . To be more specific, the injection outlet  342  is provided on the bottom surface within the space between the positions corresponding to the end of the first surface and the end of the optical element. 
         [0082]    (Storage Means) 
         [0083]    The storage means  36  stores the predetermined reference temperature t 0  to be compared with the detected temperature t 1  from the detecting means  33 .  FIG. 6  shows the detected temperature t 1  and the reference temperature t 0 . 
         [0084]    (Decision Means) 
         [0085]    The decision means  37  compares the detected temperature t 1  with the reference temperature t 0 . If the detected temperature t 1  has exceeded the reference temperature t 0 , the decision means  37  outputs the result of decision to the control means  35 . When the leading edge of the molten resin has reached the position of the detecting means  33 , the detected temperature t 1  detected by the detecting means  33  is determined as the reference temperature t 0 . 
         [0086]    (Control Means) 
         [0087]    In response to the detected temperature t 1  from the detecting means  33 , the control means  35  allows the decision means  37  to compare the detected temperature with the reference temperature. When the decision means  37  has determined that the detected temperature t 1  exceeds the reference temperature t 0 , the control means controls the charging means  32  so that charging of the cavity  31  with resin will be suspended. Further, the control means  35  controls the gas filling means  34  to start injection of compressed gas into the charged resin. The control means  35  suspends the inspection of compressed gas after the elapse of a prescribed time from the start of injection of the compressed gas.  FIG. 6  shows the operation of stopping the resin charging, and starting the injection of compressed gas, when the detected temperature t 1  has exceeded the reference temperature t 0 . 
         [0088]    When the compressed gas is injected into the charged resin, the molten resin portion that may be formed as a defect of unsightly appearance such as a hesitation mark will be handled as follows: Resin is pushed into a space having the same or greater length as the injection outlet  342  formed in the region corresponding to the second surface portions  12  located opposite the injection outlet  342  through the first surface portion  11 . Accordingly, the hollow portion  14  is formed over a wider area with sufficient margin below the region corresponding to the first surface portion  11 . The impact of the tensile stress due to the thermal shrinkage of resin is released by the formed hollow portion  14 , with the result that warping of the first surface portion  11  can be reduced. 
         [0089]    The hollow portion  14  is preferably formed over a wider range to cover the region corresponding to the second surface portions  12  because warping of the first surface portion  11  can be reduced with a high degree of reliability. 
         [0090]    Since the compressed gas is injected before resin is cooled subsequent to suspension of resin charging operation, injection of the gas is preferably started almost simultaneously with suspension, or in the range of 1 to 5 seconds after charging with resin. 
         [0091]    In response to the operation having been performed through the interface  38  by the operation means  41 , the control means  35  adjusts a prescribed time so that the updated prescribed time is stored in the storage means  36 . Adjustment of a prescribed time allows the position of the hesitation mark HM to be adjusted. 
         [0092]    In response to the instruction from the operation means  41 , the control means  35  stores the updated reference temperature t 0  in the storage means  36 . To adjust the time of suspending the resin charging operation and starting the compressed gas injection, one has only to adjust the reference temperature t 0 . The reference temperature t 0  can be determined on an empirical basis by repeating the test of manufacturing the substrate of the fθ mirror  10  and by measuring and evaluating the produced fθ mirror  10 . The reference temperature t 0  is determined on a relative basis in conformity to the material of the substrate of the fθ mirror  10 , the temperature of the heating cylinder and resin charging volume per unit time. 
         [0093]    (Material of Resin Molded Article for Optical Element) 
         [0094]    The material of the fθ mirror  10  will be described. The resin material constituting the substrate of the fθ mirror  10  is exemplified by polycarbonate, polyethylene terephthalate, polymethyl methacrylate, cyclo olefin polymer, and a resin made up of two or more of these substances. 
         [0095]    (Material of Mirror Surface Section) 
         [0096]    The following describes the material constituting the mirror surface section  13  of the fθ mirror  10 . The material constituting the mirror surface section  13  is exemplified by silicon monoxide, silicon dioxide and alumina The film can be formed by a commonly known film forming method such as a vacuum vapor deposition, sputtering or ion plating method. 
         [0097]    (Manufacturing Method) 
         [0098]    Referring to  FIG. 7 , the following describes how to manufacture the fθ mirror  10 .  FIG. 7  is a flow chart showing the step of manufacturing the fθ mirror  10 . 
         [0099]    Before the mold cavity  31  is filled with resin, the cylinder (not illustrated) of the charging means  32  is preset to reach a prescribed molten temperature. Further, the control means  35  keeps the solenoid valve  341  closed. The control means  35  controls the charging means  32  so that the screw rotates. Then the resin is injected from the nozzle  324  and is fed through the spool  323 , runner  322  and gate  321  so that the resin is charged into the cavity  31  (Step S 101 ). 
         [0100]    The cavity  31  is further filled with resin. The leading edge of the molten resin having reached the second surface portions  12  is detected by the detecting means  33 . When the decision means  37  has determined that the detected temperature t 1  detected by the detecting means  33  exceeds the reference temperature t 0  (Step S 102 : Y), the control means  35  controls the charging means  32  and suspends the operation of the cavity  31  being filled with the resin (Step S 103 ). The control means  35  controls the gas filling means  34  to open the solenoid valve  341 . This procedure ensures that the compressed gas inside the tank (not illustrated) is jetted out from the injection outlet  342  into the cavity  31 . 
         [0101]    The injection outlet  342  is located on the bottom surface  313  opposed to the second region  312  and the injection outlet  342  is opened in the direction of length. This arrangement allows compressed gas to be injected into the charged resin in the direction of length (Step S 104 ). This procedure forms a hollow portion to be formed to extend in the direction of length. Further, when the leading edge of the molten resin has reached the second surface portions  12 , the resin charging operation is suspended and the resin is filled with compressed gas. This procedure allows a hesitation mark to be formed on the second surface portions  12 , but not on the first surface portion  11 . This protects the surface precision of the first surface portion  11  against possible deterioration. 
         [0102]    The molten resin is solidified and cooled by the thermal conduction of a mold. While the molten resin is solidified and cooled, the hollow portion  14  is kept at a prescribed pressure (Step S 105 ). If the pressure is maintained, the first surface portion  11  is pressed against the first region  311 , with the result that transferability on the first surface portion  11  is improved. The mirror surface section  13  is formed in the first surface portion  11  in the process from the step of injecting the compressed gas (Step S 104 ) to the holding pressure step (Step S 105 ). This is followed by the step of removing the compressed gas from the hollow portion  14  and opening the mold to take out the fθ mirror (resin molded article)  10  (Step S 106 ). 
         [0103]    In the aforementioned Step  102 , the control means  35  receives the detected temperature t 1  as a detection signal from the detecting means  33 . When the decision means  37  has determined that the detected temperature t 1  exceeds the reference temperature, the resin charging operation is suspended and the injection of the compressed gas is started. As will be apparent from the above, the number of the detecting means  33  mounted on the bottom surface  313  (including the lateral wall surface  314 ) opposed to the second region  312  is one. A plurality of detecting means  33  can be mounted on the bottom surface  313  opposed to the second region  312 . 
         [0104]    When a plurality of detecting means  33  is mounted, suspension of the resin charging operation and start of the compressed gas filling are carried out as follows. The control means  35  controls the charging means  32  and gas filling means  34  when the detected temperature t 1  detected by a particular detecting means  33  has exceeded the reference temperature to, wherein the ordinal number of this particular detecting means  33  is preset, and is stored in the storage means  36 . When the detected temperature t 1  detected by a prescribed detecting means  33  has exceeded the reference temperature t 0 , the control means  35  controls the charging means  32  to stop resin charging operation, and controls the gas filling means  34  to adjust the start of injecting the compressed gas. When a plurality of gas filling means  34  are mounted, it is possible to improve the accuracy in determining the time for suspending the resin charging operation and starting injection of the compressed gas, and ensures a hesitation mark HM to be formed on the second surface portions  12 . 
         [0105]    In the first embodiment, a detecting means  33  is provided on the internal surface of the cavity  31  having the same range as that of the second region  312  in the direction of length, including the second region  312 . When the temperature detected by the detecting means  33  has exceeded the reference temperature, the control means  35  controls the charging means  32  and gas filling means  34 . 
         [0106]    The following describes the manufacturing device related to an example of the variation of the first embodiment with reference to  FIGS. 8 and 9 .  FIG. 8  is a functional block diagram showing the injection molding machine equipped with a detecting means  33  and a timer  39 .  FIG. 9  is a time chart showing the relationship between the detection temperature and the start of injecting the compressed gas. Since a timer  39  is provided, the detecting means  33  can be installed in the first region  311 . 
         [0107]    When the decision means  37  has determined that the detected temperature t 1  detected by the detecting means  33  exceeds the reference temperature t 0 , the control means  35  allows the timer  39  to count the time elapsed from when the detected temperature t 1  has exceeded the reference temperature t 0 . When the decision means  37  has determined that the elapsed time has exceeded a prescribed time, the control means  35  controls the charging means  32  to stop the resin charging operation. The control means  35  controls the gas filling means  34  to start the injection of the compressed gas, and to suspend gas injection after the lapse of a prescribed time from the start of compressed gas injection.  FIG. 9  shows the operation of countering the time elapsed when the detected temperature t 1  has exceeded the reference temperature t 0 , the operation of suspending the resin charging step when the time elapsed has exceeded the preset time, and the operation of starting the injection of compressed gas. 
         [0108]    One or more detecting means  33  are arranged on the internal surface of the cavity  31  having the same range as that of the first region  311  in the direction of length, without including the first region  311  for forming the first surface portion  11 .  FIG. 8  indicates a detecting means  33  arranged on the bottom surface  313  opposed to the first region  311  (ceiling surface). Here, the internal surface of the cavity  31  having the same range as that of the first region  311  in the direction of length refers to the bottom surface  313  and double lateral wall surface  314  when first region  311  is assumed as a ceiling surface. Since the detecting means  33  is arranged on the bottom surface  313 , there is no factor that may cause deterioration in the surface precision of the first surface portion  11 . 
         [0109]    When the leading edge of the molten resin is assumed to have reached the second region  312  from the first region  311 , the control means  35  suspends the resin charging operation, and initiates compressed gas injection. This procedure allows a hesitation mark HM to be formed on the second surface portions  12 . 
         [0110]    Also for example, due to some restrictions in the space for installing a detecting means  33  or the profile of the fθ mirror  10  (resin molded article), a detecting means  33  may not be provided on the internal surface of the cavity  31  having the same range as that of the second region  312  in the direction of length, including the second region  312 . In this case, the detecting means  33  can be installed on the bottom surface  313  or lateral wall surface  314  as an internal surface of the cavity  31  having the same range as the first region  311  in the direction of length. This arrangement enhances the degree of freedom in the installation of the detecting means  33 . 
         [0111]    The above-mentioned preset time is determined by a test as follows. For example, a step is taken to measure the time from the moment the decision means  37  has determined that the detected temperature t 1  detected by the detecting means  33  exceeds the reference temperature t 0 , to the moment when the leading edge of resin reaches the second range. This measurement is repeated a plurality of prescribed times. Then based on this actual measurement, the movement of the leading edge of resin (spread of resin inside the cavity  31  or movement in the direction of length) is calculated by an approximation method, whereby the above-mentioned preset time is obtained. The control means  35  ensures that the preset time having been obtained is stored in the storage means  36 . Further, in response to the operation on the operation means  41 , the control means  35  adjusts the preset time and stores it in the storage means  36 . Thus, this procedure minimizes the error between the preset time having been obtained, and the actual time before the leading edge of resin reaches the second region  312 . 
         [0112]    It is also possible to arrange such a configuration that a plurality of detecting means  33  are installed, and the traveling speed of the leading edge of resin in the direction of length can be obtained, based on detected temperatures t 1  from a plurality of detecting means  33 . In this case, the preset time is corrected in conformity to the traveling speed having been obtained, and the updated preset time is stored in the storage means  36 . The decision means  37  makes a comparison between the time elapsed from the moment the decision means  37  has determined that the detected temperature t 1  detected by the detecting means  33  exceeds the reference temperature to, and the above-mentioned updated preset time (predicted time for the leading edge of resin to reach the second region). The control means  35  controls the charging means  32  and gas filling means  34  when the decision means  37  has determined that the above-mentioned elapsed time exceeds the updated preset time. 
         [0113]    Referring to  FIG. 10 , the following describes the method for manufacturing the substrate of the fθ mirror  10  as a variation of the first embodiment.  FIG. 10  is a flow chart showing the step of manufacturing the fθ mirror  10 . 
         [0114]    The control means  35  controls the charging means  32  so that the screw rotates. Then the resin is injected from the nozzle  324  and is fed through the spool  323 , runner  322  and gate  321  so that the resin is charged into the cavity  31  (Step S 201 ). 
         [0115]    Further, the cavity  31  is charged with molten resin. The detecting means  33  detects the leading edge of the molten resin having reached the first surface portion  11 . When the decision means  37  has determined that the detected temperature t 1  detected by the detecting means  33  exceeds the reference temperature t 0  (Step S 202 : Y), the control means  35  allows the timer  39  to measure the time elapsed after this decision (Step S 203 ). When the decision means  37  has determined that the measured time exceeds the preset time (Step S 204 : Y), the control means  35  controls the charging means  32  to suspend the operation of charging the cavity  31  with resin (Step S 205 ). Then the control means  35  controls the gas filling means  34  to open the solenoid valve  341 . Then the compressed gas in the tank (not illustrated) is jetted into the cavity  31  from the injection outlet  342 . At this time, the leading edge of the molten resin has already reached the second surface portions  12 . 
         [0116]    The injection output  342  is arranged on the bottom surface  313  opposed to the second region  312  and the injection output  342  opens in the direction of length. This arrangement allows the compressed gas to be injected into the charged resin in the direction of length (Step S 206 ), whereby a hollow portion  14  extending in the direction of length in the resin is formed. When the above-mentioned elapsed time having been measured is determined to have exceeded the preset time (when the leading edge of the molten resin has reached the second surface portions  12 ), the control means  35  suspends the resin charging operation and allows the compressed gas to be injected into the resin, whereby a hesitation mark is formed on the second surface portions  12 . 
         [0117]    The molten resin is solidified and cooled by the thermal conduction with the mold. The hollow portion  14  is held at a prescribed pressure (Step S 207 ) until solidification and cooling terminate. The pressure holding step allows the first surface portion  11  to be pressed against the first region  311 . This enhances the transferability of the first surface portion  11 . This is followed by the step of removing the compressed gas from the hollow portion  14 . The mold is opened and the fθ mirror (resin molded article)  10  is taken out (Step S 208 ). 
         [0118]    The injection molding machine as a variation of the first embodiment is equipped with a detecting means  33  and a timer  339 . When the decision means  37  has determined that the detected temperature t 1  detected by the detecting means  33  exceeds the reference temperature t 0 , the timer  39  is allowed to measure the time elapsed after this decision. The control means  35  controls the charging means  32  and gas filling means  34  in conformity to the result of the measurement. 
         [0119]    The following describes the manufacturing method as a variation of the first embodiment with reference to  FIG. 11 .  FIG. 11  is a functional block diagram showing the injection molding machine equipped with a timer  39 . In response to the elapsed time measured by the timer  39 , the control means  35  controls the charging means  32  to suspend the resin charging operation, and the gas filling means  34  to start injection of compressed gas. 
         [0120]    In an injection molding machine as a variation of the present embodiment, when the time elapsed from the start of the resin charging operation has exceeded the preset time, the control means  35  controls the charging means  32  and gas filling means  34 . This procedure allows a hesitation mark HM to be formed on the second surface portions  12 . 
         [0121]    The resin charging operation can be started when the screw (not illustrated) of the charging means  32  has started, or when the control means  35  has ordered the charging means  32  to start the resin charging operation. The timer  39  counts the elapsed time. The decision means  37  determines whether or not the elapsed time having been measured has exceeded the preset time. In response to the information from the decision means  37  that the elapsed time has exceeded the preset time, the control means  35  controls the charging means  32  and gas filling means  34 . This procedure does not require use of a detecting means  33  such as a temperature sensor, and contributes to cost reductions. 
         [0122]    Referring to  FIG. 12 , the following describes the method of manufacturing the substrate of the fθ mirror  10  as another variation.  FIG. 12  is a flow chart showing a step of manufacturing the substrate of the fθ mirror  10 . 
         [0123]    The control means  35  controls the charging means  32  so that the screw rotates. Then the resin is injected from the nozzle  324  and is fed through the spool  323 , runner  322  and gate  321  so that the resin is charged into the cavity  31  (Step S 301 ). 
         [0124]    The timer  39  counts the time elapsed after the start of the resin charging operation (Step S 302 ). The cavity  31  is further charged with the molten resin. The decision means  37  determines whether or not the elapsed time having been measured has exceeded the preset time. When the decision means  37  has determined that the elapsed time having been measured exceeds the preset time (Step S 303 : Y), the control means  35  controls the charging means  32  to suspend the operation of charging the cavity  31  with resin (Step S 304 ). Then the control means  35  controls the gas filling means  34  to open the solenoid valve  341 . Then the compressed gas in the tank (not illustrated) is jetted into the cavity  31  from the injection outlet  342 . At this time, the leading edge of the molten resin has already reached the second surface portions  12 . 
         [0125]    The injection output  342  is arranged on the bottom surface  313  opposed to the second region  312  and the injection output  342  opens in the direction of length. This arrangement allows the compressed gas to be injected into the charged resin in the direction of length (Step S 305 ), whereby a hollow portion  14  extending in the direction of length in the resin is formed. When the elapsed time having been measured is determined to have exceeded the preset time by the decision means  37  (when the leading edge of the molten resin has reached the second surface portions  12 ), the control means  35  suspends the resin charging operation and allows the compressed gas to be injected into the resin, whereby a hesitation mark is formed on the second surface portions  12 . 
         [0126]    The molten resin is solidified and cooled by the thermal conduction with the mold. The hollow portion  14  is held at a prescribed pressure (Step S 306 ) until solidification and cooling terminate. The pressure holding step allows the first surface portion  11  to be pressed against the first region  311 . This enhances the transferability of the first surface portion  11 . This is followed by the step of removing the compressed gas from the hollow portion  14 . The mold is opened and the fθ mirror (resin molded article)  10  is taken out (Step S 307 ). 
       Embodiment 2 
       [0127]    Referring to  FIGS. 13 and 14 , the following describes the resin molded article for the optical element in a second embodiment of the present invention.  FIG. 13  is a plan view showing the resin molded article for the optical element.  FIG. 14  is a cross sectional view showing the resin molded article for the optical element. In the description of the resin molded article for the optical element in the first embodiment, the fθ mirror  10  has been used as a representative component. An fθ lens  20  will be used as a representative component to describe the resin molded article for the optical element in the second embodiment. 
         [0128]    Similarly to the case of the fθ mirror  10 , the fθ lens  20  is installed on the laser beam scanning optical device. While the fθ mirror  10  has a mirror surface section  13  for reflecting the laser beam, the fθ lens  20  has an optical surface section  23 . The fθ lens  20  having an optical surface section  23  has the same function as the fθ mirror  10 . The speed is converted so that the laser beam deflected at a constant angular speed by the polygon mirror  3  will have a constant linear speed on the scanned surface (photoreceptor drum  7 ). This laser beam pertaining to a semiconductor laser of gallium nitride has an oscillation wavelength of 408 mm. 
         [0129]    The fθ lens  20  is formed in a long tabular shape, and has a prescribed range H 2  in the direction of length. The fθ lens  20  includes a first surface section  21  to be provided with an optical surface section  23  for allowing passage of the optical beam received inside the a prescribed range H 2 ; a second surface section  22  arranged around the first surface section  21 ; and a hollow portion  24 . The first surface section  21  is provided on each of the upper and lower surface sides in the sheet of paper in  FIG. 4 . The first surface section  21  on the upper surface side forms a convex surface having a prescribed curved surface in the direction of width. The first surface section  21  on the lower surface side forms a concave surface having a prescribed curved surface in the direction of width. 
         [0130]    In the width in the direction of length, a prescribed range is equal to or smaller than the region of the optical surface section  23 , and the region of the optical surface section  23  is equal to or smaller than the region of the first surface section  21 .  FIG. 13  shows the region of the optical surface section  23  and the first surface section  21  which are matched with each other in the width in the direction of length. 
         [0131]    In  FIG. 13 , R 1  indicates the range of the first surface section  21  in the direction of width. R 2  denotes the range of the second surface section  22  in the direction of width. 
         [0132]    The fθ lens  20  includes a long tabular substrate, an optical surface section  23  located on the surfaces of the upper and lower surface sides of the substrate; and a hollow portion  24  inside the substrate so as to run in the direction of length, wherein the size of the hollow portion  24  in the direction of length is greater than the length of the optical surface section  23  in the direction of length, and both ends of the hollow portion  14  are formed outside both ends of the optical surface section  23  in the direction of length. This structure ensures that the tensile stress caused by shrinkage resulting from resin hardening is released into the hollow portion  24  having been formed. Thus, warping caused by shrinkage of resin at the time of resin hardening is reduced over the entire optical surface section  23 , and the surface precision is enhanced. 
         [0133]    In the resin molded article of the present embodiment, assume that the length of the optical surface section  23  in the direction of length is L 1 , the length in the direction of width is W 1 , the length of the hollow portion  14  in the direction of length is L 2 , the length in the direction of width is W 2 , the length of the substrate in the direction of thickness is W 4 , and the distance from the end of the optical surface section  23  to the end of the substrate with respect to one side in the direction of length is L 5 . It is preferred to design the structure wherein the distance L 3  from the end of the optical surface section  23  to the end of the hollow portion  24  is 0≦L 3 &lt;L 5  with respect to one side in the direction of length. The distance W 3  from the end of the optical surface section  23  to the end of the hollow portion  24  is 0≦W 3 &lt;W 2 /2 with respect to one side in the direction of width. 
         [0134]    The preferred relationship between the length W 1  of the optical surface section  23  in the direction of width and the length W 2  of the hollow portion  24  is 0.01≦W 2 /W 1 ≦1. 
         [0135]    The fθ lens  20  includes: a first molded section  25  containing a first surface section  21  as the surface thereof; and a second molded section  26  containing a second surface section  22  as the surface thereof and enclosing the first molded section  25  in the faun of a frame. The second molded section  26  has a rib  27  and end frame  28 . The rib  27  is thicker than the first molded section  25  and is formed on each side of the first molded section  25  in the direction of width perpendicular to the direction of length so as to run along the direction of length. Further, the end frame  28  is formed on each side of the first molded section  25 , and has approximately the same thickness as the first molded section  25  in such a way as to extend from the first molded section  25 . Thus, the second surface section  22  provided on the periphery of the first surface section  21  includes the surfaces (upper and lower surfaces) of the rib  27  and the surfaces (upper and lower surfaces) of the end frame  28  arranged on each side of the first surface section  21  in the direction of length. 
         [0136]    The rib  27  is provided along the first surface section  21 . This structure enhances the overall rigidity of the fθ lens  20 . The rib  27  is provided along the first surface section  21 . This allows the shape of the rib to be determined, without being restricted by the profile of the first molded section  25 . This improves the degree of freedom in the selection of the profile of the rib  27 . Thus, the hollow portion  24  can be designed in the profile that ensures easier formation, and the rib  27  can be formed, for example, to have a prescribed thickness and a prescribed width in the direction of width. Further, the rib  27  can be formed in a straight line and the hollow portion  24  can be formed in a straight line in the direction of length. Accordingly, easy formation of the hollow portion  24  is ensured by this structure. 
         [0137]    Since a hollow portion  24  is arranged inside the rib  27 , warping of the rib  27  can be reduced. This will lead to the reduction in the warping of the first molded section  25 , and will therefore protect against deterioration in the surface precision of the optical surface section  23  located on the first surface section  21  of the first molded section  25 . 
         [0138]    The hesitation mark HM is formed on the surface of the rib  27  as the second surface section  22 , or the surface of the end frame  28 . In the second embodiment, the hesitation mark HIV is formed on the second surface section  22 . This prevents unsightly appearance from being formed on the first surface section  21  to be provided with the optical surface section  23 . 
         [0139]    The following describes the injection molding machine for manufacturing the substrate of the fθ lens  20 . The basic structure of this injection molding machine is the same as the injection molding machine for manufacturing the fθ mirror  10 , and will not be described to avoid duplication. The following describes the differences in structure. 
         [0140]    (Detecting Means) 
         [0141]    One or more detecting means  33  are preferably arranged on the internal surface of the cavity  31  forming the end frame  28 . If the detecting means  33  is installed in this position, the leading edge of the molten resin spreading beyond the first surface section  21  can be directly detected and the hesitation mark HM can be formed correctly on the second surface section  22  (surface of the end frame  28 ). Further, when a plurality of detecting means  33  are installed in this position, it is possible to enhance the reliability of the hesitation mark HIM being formed on the second surface section  22 . The detecting means  33  can be arranged on the internal surface (the internal surface of the cavity  31  for forming the rib  27 ) of the cavity  31  having the same range as that of the internal surface of the cavity  31  for forming the end frame  28 . 
         [0142]    Further, the detecting means  33  can be arranged on the internal surface (the internal surface of the cavity  31  for forming the rib  27 ) of the cavity  31  having the same range as that of the internal surface of the cavity  31  for forming the first surface section  21 . In this case, when the decision means  37  has determined that the detected temperature t 1  detected by the detecting means  33  exceeds the reference temperature t 0 , the control means  35  allows the timer  39  to measure the time elapsed after this decision. In response to the information from the decision means  37  that the elapsed time has exceeded the preset time, the control means  35  controls the charging means  32  and gas filling means  34 . 
         [0143]    (Material of fθ Lens) 
         [0144]    The material of the fθ lens will be described. The resin material constituting the substrate of the fθ lens  20  is exemplified by polycarbonate, polyethylene terephthalate, polymethyl methacrylate, cyclo olefin polymer, and a resin made up of two or more of these substances. Of these, polycarbonate and cyclo olefin polymer are preferably used. 
         [0145]    (Manufacturing Method) 
         [0146]    The aforementioned manufacturing device and material are used to manufacture the substrate of the fθ lens  20 . The method for manufacturing the fθ lens  20  is basically the same as that in the first embodiment, and will not be described. 
         [0147]    The embodiments of the present invention have been described with reference to the resin molded article for the optical element. It is to be expressly understood, however, that the present invention is not restricted to the resin molded article for the optical element. For example, it goes without saying that the present invention is applicable, for example, to the resin molded article wherein a hollow portion is formed inside, the surface with a prescribed surface precision and the surface with a surface precision lower than a prescribed level are provided, and a hesitation mark formed on the surface with a surface precision lower than a prescribed level. 
       EXAMPLE 
       [0148]    The following describes the present invention with reference to the preferred Example. In the Example, the resin molded article to be manufactured is a substrate of fθ mirror  10 . A substrate of fθ mirror  10  is also used in the Comparative Example. 
         [0149]    fθ mirrors  10  were molded and manufactured using the following two patterns of molds wherein the aforementioned manufactured device and method were employed and the cavity was formed in a profile of  FIG. 3 . 
         [0150]    (Pattern 1) 
         [0000]      A=B=5.0 mm 
         [0151]    Two fθ mirrors obtained from the aforementioned mold were evaluated. It has been verified that deterioration in the profile caused by a sink mark or others is reduced on the first surface sections in both cases. The produced fθ mirrors are characterized by a high degree of surface precision. 
         [0152]    It has also been verified that, when the aforementioned mirror is applied to a scanning optical device using a laser beam having a wavelength of 408 nm, the spot can be sufficiently narrowed and high-definition image formation can be ensured. 
         [0153]    (Pattern 2) 
         [0154]    In a Comparative Example, an fθ mirror was molded and manufactured in the similar manner, using a cavity type mold wherein the first surface section was matched with the end of the optical element. A molded portion of unsightly appearance caused by hesitation was observed on the first surface section of this product. It has been demonstrated that satisfactory image formation cannot be provided by the aforementioned scanning optical device. 
       DESCRIPTION OF REFERENCE NUMERALS 
       [0155]    HM. Hesitation mark 
         [0156]    t 1 . Detected temperature 
         [0157]    t 0 . Reference temperature 
         [0158]      10 . fθ mirror 
         [0159]      11 . First surface portion 
         [0160]      12 . Second surface portions 
         [0161]      13 . Mirror surface section 
         [0162]      14 . Hollow portion 
         [0163]      20 . fθ lens 
         [0164]      21 . First surface section 
         [0165]      22 . Second surface section 
         [0166]      23 . Optical surface section 
         [0167]      24 . Hollow portion 
         [0168]      25 . First molded section 
         [0169]      26 . Second molded section 
         [0170]      27 . Rib 
         [0171]      28 . End frame 
         [0172]      31 . Cavity 
         [0173]      32 . Charging means 
         [0174]      33 . Detecting means 
         [0175]      34 . Gas filling means 
         [0176]      35 . Control means 
         [0177]      36 . Storage means 
         [0178]      37 . Decision means 
         [0179]      38 . Interface 
         [0180]      39 . Timer 
         [0181]      41 . Operation means 
         [0182]      42 . Mold 
         [0183]      311 . First region 
         [0184]      312 . Second region 
         [0185]      313 . Bottom surface 
         [0186]      314 . Lateral wall surface 
         [0187]      315 . Mirror surface forming section 
         [0188]      341 . Solenoid valve 
         [0189]      342 . Injection output