Abstract:
A die for molding disc substrates, includes, in a stamper, a to-be-held surface that diverges continuously from an inner circumferential surface of a center hole. An inner stamper holder holding the inner circumferential side of the stamper includes a cylindrical body and a claw including a cavity forming surface substantially parallel to the inner circumferential side head surface, and a holding surface diverging tapered toward the cavity holding the to-be-held surface. In the cylindrical body portion, a cylindrical surface that is opposed to the inner circumferential surface of the stamper. The die prevents the claw of the inner stamper holder from projecting from a transfer surface of the stamper and solves a problem of insufficient strength of the inner circumferential side of the stamper.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a die for molding disc substrates such as CD-ROM, CD-R, DVD-ROM, DVD-RAM, DVD-R substrates and so on and, in particular, it relates to a stamper and an inner stamper holder. 
   2. Description of the Related Art 
   Conventionally, in a die for molding disc substrates, an inner stamper holder for holding an inner circumferential side of a stamper, of the construction as shown in [0003] and FIGS. 16 and 17 of Japanese Unexamined Patent Publication No. H05-54427, is typically used. Thus, the inner circumferential side of the stamper is held to a mirror plate by a claw portion of the inner stamper holder that is projected from a surface of the stamper toward a cavity. However, the inner stamper holder, shown in FIGS. 16 and 17 of Japanese Unexamined Patent Publication No. H05-54427 mentioned above, has a problem in that the flow of melted resin from a sprue toward an information recording surface of the stamper in the cavity is restricted by the claw portion mentioned above, which reduces the cross-sectional area of the cavity. 
   In order to solve this problem, in Japanese Unexamined Patent Publication No. H05-54427 mentioned above, as shown in [0030], [0031] and FIG. 8 thereof, the inner circumferential side of the stamper is shaped to diverge in a tapered fashion by an angle ranging from 20° to 70° and the claw portion of the inner stamper holder is shaped accordingly. By shaping these elements as described above, there is an effect that the claw portion is prevented from being projected from the information recording surface of the stamper toward the cavity so that the flow of the melted resin is not obstructed. Further, Japanese Unexamined Patent Publication No. H05-54427 describes that a combination of the inner circumferential side of the stamper and the claw portion of the inner stamper holder, shown in [0032] and FIG. 9 thereof, also has an effect similar to that described above. 
   Still further, as examples similar to those described above in Japanese Unexamined Patent Publication No. H05-54427, examples shown in [0013] and FIG. 4 of Japanese Unexamined Patent Publication No. H10-302328 and in FIGS. 5 and 6 of Japanese Unexamined Utility Model Publication No. S62-80620, are known. 
   In each of the examples described above in Japanese Unexamined Patent Publication No. H05-54427, Japanese Unexamined Patent Publication No. H10-302328 and Japanese Unexamined Utility Model Publication No. S62-80620, the inner circumferential side of the stamper must be formed to a specific shape. Further, considering the fact that the stamper is moved slightly because of the difference of thermal expansion between the stamper and the inner stamper holder and the pressure of the melted resin when the disc substrates are molded, as the inner circumferential side of the stamper is fitted completely under the claw portion of the inner stamper holder in the examples described above, there is a problem in that strength of the inner circumferential side of the stamper becomes insufficient and that the service life of the stamper is shortened. 
   Further, devices for forming a center hole of the stamper shown in FIGS. 1 and 4 of Japanese Registered Utility Model No. 3032851, which form configurations around the center hole of the stamper as shown in FIGS. 3( b ) and 5( b ) thereof, are known. However, it was thought absolutely impossible to hold the stamper so that the claw portion of the inner stamper holder is not projected toward the cavity and without reshaping the configurations around the center hole of the stamper and, therefore, no attempt has been made to address this problem. 
   BRIEF SUMMARY OF THE INVENTION 
   In order to solve the above problems, it is an object of the present invention to provide a die, for molding disc substrates and aimed at improving the flow of melted resin, that prevents a claw portion of an inner stamper holder from being projected from a transfer surface of a stamper toward a cavity and solves the problem of insufficient strength of an inner circumferential side of the stamper. 
   Thus, according to the present invention, there is provided a die for molding disc substrates in which a stamper is held by at least one of a stationary die and a movable die, wherein, in the stamper, a to-be-held surface that diverges continuously from an inner circumferential surface of a center hole is formed between the inner circumferential surface and an inner circumferential side head surface by punching the center hole, an inner stamper holder that holds the inner circumferential side of the stamper is comprised of: a cylindrical body portion; and a claw portion formed around an outer circumference of an end of the cylindrical body portion, in the claw portion, a cavity forming surface that is substantially parallel to the inner circumferential side head surface and a holding surface that diverges toward the cavity in a tapered fashion and holds said to-be-held surface are formed, and, in the cylindrical body portion, a cylindrical surface that is equidistantly substantially opposed to the inner circumferential surface of the stamper, is formed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a die for molding disc substrates according to the present invention; and 
       FIG. 2  is an enlarged cross-sectional view of an important part of the die for molding disc substrates shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 1 , a die  1  for molding disc substrates is comprised of: a stationary die  2  that is attached to a stationary platen of a disc substrate molding apparatus (not shown); and a movable die  2  that is attached to a movable platen. In the stationary die  2 , a main body portion  4  is provided with a sprue bush  5 , a female cutter  6 , a back plate  7  and so on and a mirror plate  8  is disposed on the back plate  7 . Further, a sprue temperature control path  9  is formed between the sprue bush  5  and the female cutter  6  and a spiral mirror plate temperature control path  10  is formed between the back plate  7  and the mirror plate  8 . 
   On the other hand, in the movable die  3 , a main body portion  11  is provided with a cylindrical stationary sleeve  12  and an ejector sleeve  13 , a male cutter  14  and a center pin  15  are disposed in the center hole of the stationary sleeve  12 . Then, an inner stamper holder  16  can be mounted around the stationary sleeve  12 . Further, a back plate  17  is disposed on said main body portion  11 , and around the outside of said inner stamper holder  16  and a mirror plate  18  is disposed on the back plate  17 . In this embodiment, the mirror plate  18  has a thickness of 20 mm. A stamper  20 , the inner circumferential side  20   a  of which is held by said inner stamper holder  16  and the outer circumferential side of which is held by an outer circumferential stamper holder  21 , respectively, is mounted on a surface  19  of the mirror plate  18 . Further, a male cutter temperature control path  22  is formed inside the male cutter  14  and a plurality of circular mirror plate temperature control paths  23  are formed between the back plate  17  and the mirror plate  18 . In this embodiment, elements constituting the die such as the mirror plate  18 , the inner stamper holder  16  and the like are made of 13 Cr stainless steel containing chromium or, more specifically, SUS420J2 having a thermal expansion coefficient of about 11.5×10 −6 /° C. 
   Then, a cavity  24  having a variable volume is formed by fitting an outer circumferential portion of the mirror plate  8  of said stationary die  2  into an inner circumferential portion of the outer stamper holder  21  of said movable die  3 . A sprue  25  is formed in the center of said sprue bush  5  and passes through the cavity  24  so that melted resin can be injected from an injection device (not shown) via said sprue  25  to fill the cavity  24 . 
   Next, the stamper  20  used in the present invention will be described with reference to  FIG. 2 . The stamper  20  is formed of nickel or nickel alloys having a thermal expansion coefficient of about 12.8×10 −6 /° C. A center hole  31  of the stamper  20  is formed by punching from the side of a head surface  20   b  or a transfer surface  35  toward a back surface  20   c  thereof, as described in said Japanese Registered Utility Model No. 3032851. As shown in  FIG. 2 , in the centerhole  31  of the stamper  20  that is formed by punching, an inner circumferential surface  32  that is perpendicular to the head surface  20   b  and the back surface  20   c  and a to-be-held surface  33  that diverges continuously from said inner circumferential surface  32  toward the surface  20   b  are formed. Then, on the head surface  20   b  of the stamper  20 , an inner circumferential side head surface  34  and the transfer surface  35  are formed continuously from said to-be-held surface  33 . 
   In the present invention, as the to-be-held surface  33  that diverges continuously from the inner circumferential surface  32  can be formed by punching, the special processing to allow the inner stamper holder  16  to hold the inner circumferential side  20   a  of the stamper  20  as described above in Japanese Unexamined Patent Publication No. H05-54427, Japanese Unexamined Patent Publication No. H10-302328 and Japanese Unexamined Utility Model Publication No. S62-80620 is not necessary. In this connection, when the center hole  31  of the stamper  20  is formed by punching, burrs are formed on the back surface  20   c  of the stamper  20  and, if the burrs obstruct the attachment of the stamper  20 , the burrs should be removed by polishing. If the burrs are too small to obstruct the attachment of the stamper  20 , an annular groove may be formed inside the mirror plate  18  so as to insert the burrs. Here, it is to be noted that the stamper  20  is not limited to that described above, in which the to-be-held surface  33  is formed by punching the center hole  31 , and the to-be-held surface  33  may be formed by separate processing. In any case, in the stamper  20  of the present invention, the to-be-held surface  33  that diverges between the inner circumferential surface  32  and the inner circumferential side head surface  34  is formed and the inner circumferential side  20   a  of the stamper  20  has a specified thickness. Therefore, in contrast to the stampers in which the entire inner circumferential portion is formed in a tapered fashion or a thin walled portion is formed as described above in Japanese Unexamined Patent Publication No. H05-54427, Japanese Unexamined Patent Publication No. H10-302328 and Japanese Unexamined Utility Model Publication No. S62-80620, the inner circumferential side  20   a  of the stamper  20  is not likely to be damaged. 
   Next, the inner stamper holder  16  to hold the inner circumferential side  20   a  of the stamper  20  will be described with reference to  FIG. 2 . The inner stamper holder  16  is a member that has a specified length in the direction of the axis line A and a cylindrical body portion  41  of the inner stamper holder  16  has a claw portion  42  on the outer circumference at the end facing the cavity. The end face of the cylindrical body portion  41  and the claw portion  42  facing the cavity constitutes a cavity forming surface  43  that is substantially parallel to the surface  34  of the held stamper  20 , adjacent to the inner circumferential surface. 
   The cavity forming surface  43  at the claw portion  42  is configured so that its outer circumferential portion  44  is projected from a cylindrical surface  47  of the cylindrical body portion  41  in the direction perpendicular to the axis line A by 50 μm at room temperature (20° C.). Further, the dimension of a base portion  45  of the claw portion  42  in the direction of the axis line A, which corresponds to a boundary portion between the cylindrical body portion  41  of the inner stamper holder  16  and the claw portion  42 , is 100 μm at room temperature. The claw portion  42  is formed so that it is connected with the cylindrical body portion  41  at a connection point  45   a  on said base portion  45  on the side opposite to the cavity. Then, a holding surface  46  is formed so that it diverges in a tapered fashion toward the cavity so as to connect the connection point  45   a  on said base portion  45  with the outer circumferential portion  44  of the cavity-forming surface  43 . Then, a connecting surface  48  having a concave arcuate cross section is formed on the holding surface  46  on the side opposite to the cavity so that the holding surface  46  is connected to the cylindrical surface  47  smoothly at the connection point  45   a  on the base portion  45 . Thus, the majority of said holding surface  46  consists of a tapered surface and is connected to the cylindrical surface  47  via the connecting surface  48 . Further, said cylindrical surface  47  is formed perpendicularly to said cavity forming surface  43  to face the inner circumferential surface  32  of said stamper  20  equidistantly. 
   Therefore, according to the present invention, the to-be-held surface  33  of the stamper  20  that is formed by punching can be held by the holding surface  46  of the claw portion  42  that diverges in a tapered fashion and the flowability of resin can be improved without special processing of the to-be-held surface  33  of the stamper  20 . Further, as the holding surface  46  of the claw portion  42  is formed so that it has an arcuate cross section, the damage of the inner circumferential side of the stamper  20  due to thermal expansion of the stamper  20  and the mirror plate  18  at the time of molding can be prevented further. Here, it is to be noted that the holding surface  46  may be formed so that it has a concave or convex arcuate cross section as a whole. 
   Then, the holding surface  46  of the claw portion  42  is nitrided so that a layer of chromium nitride is formed thereon. Though this chromium nitride layer is aiming at improving wear resistance, it is not essential to the present invention. The chromium nitride layer may also be formed on the cavity forming surface  43  and the cylindrical surface  47 . 
   In this connection, it is desirable that the claw portion  42  is formed so that the outer circumferential portion  44  is projected from the cylindrical surface  47  of the cylindrical body portion  41  in the direction perpendicular to the axis line A by 30–60 im at room temperature. Further, it is desirable that the dimension of the base portion  45  of the claw portion  42  in the direction of the axis line A is 50–150 im at room temperature. Thus, by forming the claw portion  42  into the shape described above, the flowability of the resin can be improved and the melted resin can be prevented from flowing between the stamper  20  and the inner stamper holder  16  and forming burrs on molded substrates. In addition, the stamper  20  can be prevented from coming off and it is possible to increase the number of times that the inner stamper holder  16  and the stamper  20  can be used. Here, the connecting surface  48  having the arcuate cross section is not essential to the present invention. Though it is desirable that all the elements constituting the cavity forming surface  43  of the inner stamper holder  16  are disposed on an identical plane, the cavity forming surface  43  at the claw portion  42  may be disposed at a height that is slightly different from the cavity forming surface  43  at the cylindrical body portion  41  so long as the difference of the height does not obstruct the flow of the melted resin. 
   Next, an operation for attaching the stamper  20  and the inner stamper holder  16  to the movable die  3  will be described with reference to  FIGS. 1 and 2 . When the stamper  20  is attached to the mirror plate  18  of the movable die  3 , first, the cylindrical body portion  41  of the inner stamper holder  16  is inserted through the center hole  31  of the stamper  20 . Then, an end of the cylindrical body portion  41  of the inner stamper holder  16  on the side opposite to the cavity, around which the stamper  20  is inserted, is inserted into an annular clearance between the stationary sleeve  12  and the mirror plate  18 . Then, an end face  49  of the inner stamper holder  16  on the side opposite to the cavity is positioned by using a rotating member (not shown) so that the end face  49  is held to an abutting surface  12   a  of the stationary sleeve  12 . As a result of the above operation, the relative position of the inner stamper holder  16  is determined with respect to the mirror plate  18  and the stamper  20 . 
   In this embodiment, the cavity forming surface  43  at the claw portion  42  of the inner stamper holder  16  is designed to be positioned so that it is projected with respect to the inner circumferential side head surface  34  of the attached stamper  20  toward the cavity by 15 im at room temperature (20° C.). This is because the thermal expansion of the mirror plate  18  and the stamper  20  at the time of molding is greater than that of the inner stamper holder  16 . Thus, the cavity forming surface  43  at the claw portion  42  is designed to lie at the height substantially identical to that of the inner circumferential side head surface  34  of the stamper  20 , adjacent to the inner circumferential surface at the time of molding, as indicated by a chain double-dashed line in  FIG. 2 . Though the height of the projection of the cavity forming surface  43  at the claw portion  42  with respect to the inner circumferential side head surface  34  of the stamper  20  may vary depending on the thickness of the mirror plate  18  and the stamper  20  and the temperature difference between the mirror plate  18  and the inner stamper holder  16  at the time of molding, it is typically designed to be projected toward the cavity or the other die by 5–25 im at room temperature and the difference in height stays within 10 im at the time of molding. Therefore, a step height on the molded products that is created between a surface formed by the inner circumferential surface of the stamper  20  and a surface formed by the cavity forming surface  43  of the inner stamper holder  16  can be eliminated or becomes very small. 
   Though an example in which the stamper  20  and the inner stamper holder  16  are provided in the movable die  3  are shown in the embodiment described above, the stamper  20  may be held by the inner stamper holder  16  in at least one of the stationary die  2  and the movable die  3 . When the stamper  20  is held in the stationary die  2 , the inner stamper holder  16  is attached between the female cutter  6  and the mirror plate  8 . 
   Next, an operation for molding disc substrates by using the disc substrate molding die  1  according to the present invention will be described. In this embodiment, it is assumed that DVD-R substrates are molded from polycarbonate resin and the temperature of the melted resin is about 360° C. at the nozzle when it is injected. Further, the temperature of the mirror plate  18  in the movable die  3  is controlled to about 120° C. by means of a temperature control media flowing through the mirror plate temperature control path  23  and the temperature of the inner stamper holder  16  is controlled to about 85° C. by means of a temperature control media flowing through the male cutter temperature control path  22 . Therefore, as the expansion of the stamper  20  and the mirror plate  18  toward the cavity is greater than that of the inner stamper holder  16  toward the cavity, the inner circumferential side head surface  34  of the stamper  20  and the cavity forming surface  43  at the claw portion  42  of the inner stamper holder  16  lie at a substantially identical height at the time of molding. At this time, as the holding surface  46  diverges in a tapered fashion toward the cavity and, on the side of the base portion  45 , the connecting surface  48  having the arcuate cross section is formed to be connected with the cylindrical surface  47 , the inner circumferential side  20   a  of the stamper  20  is not damaged even if the stamper  20  expands. Further, the distance between the holding surface  46  of the inner stamper holder  16  and the to-be-held surface  33  of the stamper  20  is maintained to be sufficiently small to prevent the inflow of the melted resin. 
   Then, when the melted resin is injected from the injection device (not shown), via the sprue, to fill the cavity  24 , as the claw portion  42  of the inner stamper holder  16  is not projected toward the cavity, the cross-sectional area where the melted resin flows is increased so that the cavity  24  can be filled with the melted resin satisfactorily. Further, according to the present invention, surfaces of the disc substrates on the side of the stamper (surfaces on which signals are recorded) can be molded flat. Therefore, when substrates such as CD-ROM, CD-R substrates, and the like, are molded, decorations and the like can be printed on the entire surface of the disc substrates. Further, when substrates such as DVD-R substrates, and the like, are molded, a pair of disc substrates can be bonded together over the entire surfaces.