Abstract:
A lens forming metal mold comprising: a first metal mold portion that forms a first lens surface of a lens; and a second metal mold portion that forms a second lens surface of the lens. An air vent groove is formed on at least one contacting surface of a contacting surface of the first metal mold portion contacting with the second metal mold portion and a contacting surface of the second metal mold portion contacting with the first metal mold portion. A convex surface serving to form a concaved portion on the lens for accommodating a burr caused by the air vent groove is formed at least around the air vent groove on an outer circumference forming surface of the at least one contacting surface on which the air vent groove is formed, the outer circumference forming surface serving to from a tubular surface of the lens.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a lens forming metal mold, a lens forming method, a lens, and a pickup device. 
       BACKGROUND 
       [0002]    An optical disc drive for reading information from an optical disk, such as CD or DVD, has a pickup device. The pickup device includes at least one laser light source and a lens for focusing a laser light beam emitted from the laser light source on an optical disk. 
         [0003]    In general, the lens is a resin molded product formed by mold injection. One example of metal molds for forming a lens is shown in Japanese Patent Provisional Publication No. 2008-126611A (hereafter, referred to as Patent Document 1). 
         [0004]    As shown in the patent document 1, when a lens is formed through mold injection, it is necessary to form an air vent groove on the metal mold for pushing out an air inside of the metal mold. And, a burr is formed on the lens by the resin injected into the metal mold and reaching to the air vent groove. 
         [0005]    Such a burr of the lens can interfere with a lens holder of the pickup device, and can be an obstacle for positioning the lens. Therefore, conventionally, a cutting process for removing the burr was performed after forming the lens. 
       SUMMARY 
       [0006]    The present invention is achieved to solve the above described problem. The present invention is advantageous in that it provides a lens forming metal mold and a lens forming method for forming a lens which can be attached to the lens holder without performing a burr removing process after forming, and the lens formed by such a method and a pickup device on which the lens is mounted. 
         [0007]    According to an aspect of the invention, there is provided a lens forming metal mold comprising: a first metal mold portion that forms a first lens surface of a lens; and a second metal mold portion that forms a second lens surface of the lens. An air vent groove is formed on at least one contacting surface of a contacting surface of the first metal mold portion contacting with the second metal mold portion and a contacting surface of the second metal mold portion contacting with the first metal mold portion. A convex surface serving to form a concaved portion on the lens for accommodating a burr caused by the air vent groove is formed at least around the air vent groove on an outer circumference forming surface of the at least one contacting surface on which the air vent groove is formed, the outer circumference forming surface serving to from a tubular surface of the lens. 
         [0008]    With this configuration, the burr extends from the concaved portion of the tubular surface of the lens. Thus, a tip of the burr does not protrude greatly outward, and the burr does not interfere with the lens holder of the pickup device, even if the burr is not removed. Therefore, the lens formed with the metal mold of the present invention can be attached to the lens holder without removing the burr. 
         [0009]    In at least one aspect, the convex surface may be formed in a shape which is along the tubular surface of the lens. 
         [0010]    In at least one aspect, that the tubular surface of the lens may be a cylindrical surface. 
         [0011]    In at least one aspect, a height of the convex surface of the outer circumference forming surface may be greater than or equal to 1.5 times of a thickness of the air vent groove. 
         [0012]    In at least one aspect, the air vent groove may be formed on both of the first metal mold portion and the second metal mold portion. 
         [0013]    According to another aspect of the invention, there is provided a lens forming method, comprising: forming a lens by injecting a molten resin material into one of the above described lens forming metal molds. 
         [0014]    According to another aspect of the invention, there is provided a lens produced by the above described lens forming method. 
         [0015]    According to another aspect of the invention, there is provided a pickup device including one of above described lenses and a holder for holding the lens. 
         [0016]    As described above, according to the present invention, the lens forming mold and lens forming method for forming a lens which can be attached to the lens holder without performing a burr removing process after forming are realized, and the lens and the pickup device are realized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a sectional view of a lens and a metal mold for forming a lens, according to an embodiment of the present invention. 
           [0018]      FIG. 2  shows a lens which has been formed and placed below an upper metal mold portion and a concaved portion of the upper metal mold portion. 
           [0019]      FIG. 3  is an explanatory illustration for explaining a situation where the lens is mounted on a lens holder. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0020]    Hereinafter, an embodiment of the present invention is explained with reference to the accompanying drawings. In the embodiment, a lens  1  is made of a plastic and manufactured through mold injection, in which a molten resin is injected into a metal mold  200 .  FIG. 1  shows a sectional view of the lens  1  and a metal mold for forming the lens  1 . 
         [0021]    The lens  1  is a lens such that a first lens surface R 1  is formed on one of surfaces of the lens (downward in the figure) and a second lens surface R 2  is formed on the other one of the surfaces of the lens (upward in the figure). Both of the first lens surface R 1  and the second lens surface R 2  are convex surfaces. The lens  1  functions as a convex lens. 
         [0022]    The lens  1  is used for a pickup device for reading out information from an optical disk, or for recording information on an optical disk. The pickup device causes a laser beam from a laser light source to enter the lens  1 , and irradiates the laser beam collected by the lens  1  on an optical disk. When data recorded on an optical disk is read out, the laser beam enters a photodetector of the pickup device through the lens  1 , after reflecting on the optical disk. Namely, the laser beam reflected on the optical disk is eventually read by the photodetector. Here, the laser light source and the photodetector are placed on a side of the first lens surface R 1 . On the other hand, the optical disk is placed on a side of the second lens surface R 2 . 
         [0023]    Next, a method of forming the lens  1  is explained. As shown in  FIG. 1 , the metal mold  200  for forming the lens  1  includes a lower metal mold portion  210  and an upper metal mold portion  220 . The lower metal mold portion  210  includes an outer metal mold (down)  211  and an inner metal mold (down)  212 . A hole  211   a  is formed in the outer metal mold (down)  211 , and the inner metal mold (down)  212  is placed inside the hole  211   a  of the outer metal mold (down)  211 . Further, the upper metal mold portion  220  includes an outer metal mold (up)  221  and an inner metal mold (up)  222 . A hole  221   a  is formed in the outer metal mold (up)  221 , and the inner metal mold (up)  222  is placed inside the hole  221   a  of the outer metal mold (up)  221 . Both of an upper surface  211   b  of the outer metal mold (down)  211  and a lower surface  221   b  of the outer metal mold (up)  221  are surfaces perpendicular to an optical axis AX of the lens  1 . The upper surface  211   b  of the outer metal mold (down)  211  and the lower surface  221   b  of the outer metal mold (up)  221  are in contact with each other. Therefore, the outer metal mold (up)  221  is supported on the outer metal mold (down)  211 . 
         [0024]    A concaved portion  212   a  for forming the first lens surface R 1  of the lens  1  is formed on the inner metal mold (down)  212 . Further, a concaved portion  222   a  for forming the second lens surface R 2  of the lens  1  is formed on the inner metal mold (up)  222 . 
         [0025]    In the embodiment, the inner metal mold (down)  212  is movable with respect to the outer metal mold (down)  211  in the direction along the optical axis AX of the lens  1 . Similarly, the inner metal mold (up)  222  is movable with respect to the outer metal mold (up)  221  in the direction along the optical axis AX of the lens  1 . The lenses  1  with various thicknesses can be formed by moving the inner metal mold (down)  212  and the inner metal mold (up)  222  along the optical axis AX. 
         [0026]    Further, both of an inner circumferential surface of the hole  221   a  of the outer metal mold (up)  221  and an inner circumferential surface of the hole  211   a  of the outer metal mold (down)  211  are placed on a same cylindrical surface centered by the optical axis AX. With such a configuration, both of the holes  211   a  and  221   a  can be formed at once by overlapping the outer metal mold (up)  221  with the outer metal mold (down)  211  and drilling them with a drill. The inner surfaces of the holes  211   a  and  221   a  can be concentric cylindrical surfaces with extremely high precision by lap finishing the holes  211   a  and  221   a  while overlapping the outer metal mold (up)  221  with the outer metal mold (down)  211 . In this manner, since the holes  211   a  and  221   a  are concentrically formed with extremely high precision, the inner metal mold (up)  222  and the inner metal mold (down)  212  are positioned with extremely high precision in a state in which the inner metal mold (up)  222  is attached to the hole  221   a  and the inner metal mold (down)  212  is attached to the hole  211   a.  Therefore, the concaved portion  222   a  of the inner metal mold (up)  222  and the concaved portion  212   a  of the inner metal mold (down)  212  are assembled to the outer metal mold (up)  221  and the outer metal mold (down)  211 , respectively, without eccentricity. Consequently, the formed lens  1  is a lens such that an optical axis of the first lens surface R 1  coincides with an optical axis of the second lens surface R 2  with extremely high precision. 
         [0027]    As described above, the lens  1  of the embodiment is formed through mold injection. Therefore, an air vent groove  223  for communicating an inner portion of the metal mold  200  with an outer portion of the metal mold  200  is formed on the bottom surface of the outer metal mold (up)  221  so as to discharge an air inside of the metal mold  200  during formation. When the molten resin is injected inside of the metal mold  200  during mold injection, the air inside of the metal mold  200  is pushed out to the outside of the metal mold through the air vent groove  223  by the injected resin. Here, a thickness T (vertical dimension in  FIG. 1 ) of the air vent groove  223  is about 0.01 to 0.02 mm. 
         [0028]    Burrs formed on the lens  1  are explained below.  FIG. 2  shows lens  1  placed below the upper metal mold portion  220  and the concaved portion  222   a  ( FIG. 1 ) of the upper metal mold portion  220 , after formation, in the embodiment. As described above, since the air vent groove  223  is formed on the metal mold  200 , a burr B is formed on the lens  1  when the resin reaches to the air vent groove  223  during formation. 
         [0029]    As shown in  FIGS. 1 and 2 , the lens  1  of the embodiment has a shape such that a ring-shaped flange portion  11  is formed outside the first lens surface R 1  and the second lens surface R 2  in a radial direction. The lens  1  is configured to be held by a lens holder provided in the pickup device at the flange portion  11 . The lens holder has a cylindrical surface having substantially the same diameter as a diameter of an outer circumferential surface  12  of the flange portion  11  of the lens  1 , and positioning of the lens  1  in the radial direction is performed by contacting the cylindrical surface of the lens holder with the outer circumferential surface  12  of the flange portion  11  of the lens  1 . 
         [0030]    As shown in  FIG. 2 , in the embodiment, a convex surface  221   d  protruding toward inside of the lens  1  in the radial direction is formed at a portion around the air vent groove  223  of an outer circumference forming surface  221   c  for forming the outer circumferential surface  12  of the lens  1 , on the outer metal mold (up)  221 . Namely, on the outer circumferential surface  12  of the lens  1 , a portion  12   a  closer to the burr B is a concaved portion having a smaller diameter than a diameter of a principal portion  12   b  of the outer circumferential surface  12 . Therefore, the burr B extends along the air vent groove  223  from the concaved portion  12   a  which is placed at an inner side in the radial direction than the principal portion  12   b  of the outer circumferential surface  12 . Consequently, a length L 1  which is a length from the optical axis AX of the lens  1  to a tip of the burr B becomes shorter than a length L 2  which is a length from the optical axis AX to the principal portion  12   b  of the outer circumferential surface  12  (namely, a virtual cylindrical surface S (a dashed line portion in the figure) which is formed if the principal portion  12   b  of the outer circumferential surface  12  is extended does not cross with the burr B). When the lens  1  is attached to the lens holder of the pickup device, the burr does not interfere with the cylindrical surface of the lens holder. Thus the lens  1  can be accurately positioned to the lens holder. 
         [0031]    Here, it is preferable that a size of the convex surface  221   d  of the metal mold  200  is as small as possible. Therefore, the convex surface  221   d  is formed in a neighborhood of the air vent groove  223 , and has an arc like shape along the outer circumference forming surface  221   c  which is a cylindrical surface. 
         [0032]    Further, in the embodiment, a height D of the convex surface  221   d  is set to greater than or equal to 1.5 times of the thickness T (see  FIG. 1 ) of the air vent groove  223  (for example, D=0.03 mm) With this configuration, a depth of the concaved portion of the lens  1 , namely, a distance between the virtual cylindrical surface S and the concaved portion  12   a  (a difference between a radius L 2  at the principal portion  12   b  of the outer circumferential surface of the lens  1  and a radius L 3  at the concaved portion  12   a  of the lens  1 ) is substantially equal to the height D of the convex surface  221   d  so that it is ensured that the burr B is not crossed with the virtual cylindrical surface S (the dashed line portion in the figure). 
         [0033]    Further, in the embodiment, the air vent groove is formed only on the outer metal mold (up)  221 . However, the air vent groove can be formed only on the outer metal mold (down)  211 , or the air vent groove can be formed on both of the outer metal mold (up)  221  and the outer metal mold (down)  211 . 
         [0034]      FIG. 3  illustrates a situation where the lens  1  is mounted on a lens holder  110  provided in a pickup device  100 . The pickup device  100  is configured to cause the laser beam to be incident on the lens  1  so that the laser beam converged by the lens  1  is incident on the optical disc. As shown in  FIG. 3 , the lens holder  110  has a holding surface  111  on which the lens  1  is placed. The lens  1  is fixed on the lens holder  110  by placing the flange portion (mounting surface)  11  of the lens  1  on the holding surface  111  of the lens holder  110  and, subsequently, injecting an adhesive Ab into a corner  12 ′ of the mounting surface of the lens  1 . As described, the optical disk is in close proximity to the second lens surface R 2  of the lens  1 . Further, a cap  40  made with soft resin is attached at a corner  22  of the lens  1  so as not to damage the optical disk by contacting the optical disk rotating at a high speed with the corner  22  at the side of the second lens surface R 2  of the lens  1 . When a cap  40  is attached to the lens  1 , an upper surface  41  of the cap  40  is placed at a higher position than the position of the surface of the second lens surface R 2  of the lens  1  (namely, closer to the optical disk). Therefore, even if the optical disk is inclined to the second lens surface R 2  of the lens  1 , the optical disk does not contact with the lens  1 . Further, a surface for detecting inclination  21  (a third surface portion) which is a ring-shaped surface perpendicular to the optical axis AX of the lens  1  is formed around a periphery of the second lens surface R 2 . During attachment of the lens  1  to the lens holder  110 , it can be detected whether the lens  1  is inclined with respect to the lens holder  110  based on an angle of reflected light by irradiating a laser beam to the surface for detecting inclination  21 . An inclination of the lens  1  is adjusted based on a result of the above detection. 
         [0035]    This application claims priority of Japanese Patent Application No. P2010-049026, filed on Mar. 5, 2010. The entire subject matter of the applications is incorporated herein by reference.