Stamper plate attaching/detaching device of injection mold for optical disc substrate

Abstract: A stamper plate attaching/detaching device of an injection mold for an optical disc substrate is used for a mold wherein the stamper plate is supported in a cavity which is formed by a cavity side disc cavity plate, a movable disc cavity plate, and a cavity ring. The stamper plate attaching/detaching device is equipped with: a section for holding the outer circumferential rim of the stamper plate by means of the inner circumferential rim of the cavity ring, which inner circumferential rim is made larger than the outer diameter of the stamper plate; a plurality of pairs of permanent magnets which have magnetic poles in parallel to the central line and which are buried, with phase differences, on the same radial line of the cavity side disc cavity plate and the movable disc cavity plate; and rotating means for giving the cavity ring a relative rotation between a first angle position and a second angle position in relation to the cavity side disc cavity plate and the movable disc cavity plate.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stamper plate attaching/detaching device of an injection mold for an optical disc substrate for a CD-ROM or the like. 2. Description of the Related Art FIG. 4 is a cross-sectional view of an essential section of an optical disc substrate injection mold which employs a conventional cavity ring. A stamper plate attaching/detaching device for the optical disc substrate injection mold holds the stamper plate by using the cavity ring and a means for holding the central hole of the stamper plate. A hole 5 for receiving a means 2 for holding the inner circumferential rim of the stamper plate is provided at the center of a cavity side disc cavity plate 4. The cavity side disc cavity plate 4 is fixed to a cavity side base plate 13. A sprue bush 8, which has a resin passage 9 at the center thereof, is provided at the center of the aforesaid members. To attach a stamper plate 1, the cylindrical body of the stamper plate holding means 2 is inserted in the central hole in the disc-shaped stamper plate 1 to receive the stamper plate 1 by a flange 3 of the stamper plate holding means 2. The stamper plate holding means 2 is inserted in the central hole 5 of the cavity side disc cavity plate 4 and fixed by a publicly known method. Then, a cavity ring 6 is installed to the cavity side disc cavity plate 4 by mounting bolts 7. This procedure is reversed when detaching the stamper plate 1. A movable disc cavity plate 10 is fixed to a movable base plate 14. A punch 12 for punching a central hole in a disc substrate is inserted in a central hole 11 of the movable disc cavity plate 10. The conventional device stated above is designed to mechanically attach or detach the outer circumferential rim of the stamper plate 1 by installing or removing the cavity ring 6 by installing or removing the mounting bolts 7; it is extensively used because it enables the stamper plate 1 to be securely attached or detached, thus leading to high reliability. The stamper plate 1, however, must be attached or detached with the mold installed in a limited space in a molding machine. This makes it extremely difficult and also dangerous to detach or attach the stamper plate. Hence, there has been high demand for improvement. To solve the problem stated above, there has been proposed an injection mold for optical disc substrates which is equipped with a vacuum suction type stamper plate holder (refer, for example, to Japanese Patent Publication No. 2-60502). In this type of metal mold, the stamper plate can be easily removed from the disc cavity plate by stopping vacuum suction. This metal mold, however, has the following shortcoming: the cavity of the metal mold for injection-molding optical disc substrates is frequently put in a vacuum state by vacuum suction before filling the cavity with melted resin in order to improve the uniformity of the density of the melted resin charged in the cavity. In this case, the rear surface of the stamper plate and the cavity are vacuumized to the same level and the suction force on the rear surface of the stamper plate does not work. This leads to a danger in that the stamper plate comes off a cylindrical bush due to the vibration of the molding machine operating at high speed or a shock applied to the molding machine. Another problem is the need for driving a vacuum pump at all times as long as a stamper plate is mounted even when molding operation is interrupted. There is also a magnetic attraction type metal mold for molding disc substrates as disclosed in Japanese Patent Laid-Open No. 5-185475 and in document EP-A-0 537 953. This type of metal mold makes use of the fact that the stamper plate is made of nickel featuring permeability; a solenoid coil electromagnet or magnet shaped like a disc is buried in the entire surface of a stamper plate mounting surface of the disc cavity plate on the stamper plate side so as to magnetically attract the stamper plate. The design is advantageous in that the structure of the metal mold is simpler since it does not use a vacuum pump. This type of metal mold, however, has the following shortcomings. Firstly, the disc cavity plate uses martensite-based stainless steel which is subjected to heat treatment to hardness of H R C 55 to 60 and also to corrosion-resistance treatment to hardness of H V 1800 or more in order to protect the surface of the disc cavity plate from scratches and corrosion. In addition, the roughness of the surface must be finished to 10 nm or less. Therefore, it is technically extremely difficult to satisfy the above requirements in embedding a soft magnet material in the surface of the disc cavity plate. Secondly, the heat controlling techniques including the one for maintaining uniform cooling rate for the disc cavity plate is as important as the aforesaid selection of the material in ensuring the quality of the disc substrates. It is extremely difficult to assure satisfactory molding functions in the presence of the magnet or other between the cooling water channel provided at the back surface of the disc cavity plate and the surface of the disc cavity plate. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a stamper plate attaching/detaching device of an injection mold for an optical disc substrate, which device features a simple construction and which permits easy installation and removal of the stamper plate without impairing the basic functions of a molding machine. To this end, according to one aspect of the present invention, there is provided a stamper plate attaching/detaching device of an injection mold for an optical disc substrate for supporting the stamper plate in a cavity which is formed by a cavity side disc cavity plate, a movable disc cavity plate, and a cavity ring; the stamper plate attaching/detaching device being equipped with: a section for holding the outer circumferential rim of the stamper plate by means of the inner circumferential rim of the cavity ring, which inner circumferential rim is made larger than the outer diameter of the stamper plate; a plurality of pairs of permanent magnets which have magnetic poles oriented in parallel to the central line and which are buried, with phase differences, along the same radial line of the cavity side disc cavity plate and the movable disc cavity plate; rotating means for giving the cavity ring a relative rotation between a first angle position and a second angle position in relation to the cavity side disc cavity plate and the movable disc cavity plate; and an attractive force generating structure designed to cause an attractive force to be generated between the permanent magnets of the cavity side disc cavity plate and the cavity ring but cause no attractive force to be generated between the permanent magnets of the movable disc cavity plate and the cavity ring the cavity ring is in the first angle position; and to cause no attractive force to be generated between the permanent magnets of the cavity side disc cavity plate and the cavity ring but cause the attractive force to be generated between the permanent magnets of the movable disc cavity plate and the cavity ring when the cavity ring is in the second angle position. In a preferred form, the stamper plate attaching/detaching device further comprises a stamper plate central hole holding means which holds the central hole rim of the stamper plate, which is pressed by the cavity ring against either disc cavity, against the disc cavity. In another preferred form, the cavity ring is composed of a ferromagnetic material; it is provided with a hole which prevents the attractive force from being generated when it confronts any pair of the permanent magnets. In a further preferred form, the means for rotating the cavity ring is a rotating handle which is manually operated. In yet another preferred form, the magnet material for the permanent magnets is of a neodymium (Ne-Fe-B) type, a samarium cobalt (Sm-Co) type, or an alnico (Al-Ni-Co) type. According to another aspect of the present invention, there is provided a stamper plate attaching/detaching device of the injection mold for the optical disc substrate for supporting the stamper plate in a cavity which is formed by a cavity side disc cavity plate, a movable disc cavity plate, and a cavity ring; the stamper plate attaching/detaching device being equipped with: a section for holding the outer circumferential rim of the stamper plate by means of the inner circumferential rim of the cavity ring, which inner circumferential rim is made larger than the outer diameter of the stamper plate; a plurality of pairs of permanent magnets which have magnetic poles oriented in parallel to the central line and which are buried along the same radial line of the cavity ring; and rotating means for giving the cavity ring a relative rotation between a first angle position and a second angle position in relation to the cavity side disc cavity plate and the movable disc cavity plate; wherein the cavity side disc cavity plate and the movable disc cavity plate are provided with a structure designed to cause an attractive force to be generated between the cavity side disc cavity plate and the permanent magnets but to cause no attractive force to be produced between the movable disc cavity plate and the permanent magnets when the cavity ring is in the first angle position; and to cause no attractive force to be generated between the cavity side disc cavity plate and the permanent magnets but to cause the attractive force to be generated between the movable disc cavity plate and the permanent magnets when the cavity ring is in the second angle position. In a preferred form, the stamper plate attaching/detaching device further includes a stamper plate central hole rim holding means which holds the central hole rim of the stamper plate, which is pressed by the cavity ring against either disc cavity, against the disc cavity. In another preferred form, the stamper plate attaching/detaching device further includes a ferromagnetic section which generates an attractive force when it confronts the permanent magnets of the cavity ring of the cavity side disc cavity plate and the movable disc cavity plate and a hole or a nonmagnetic section which prevents the attractive force from being generated when the cavity side disc cavity plate and the movable disc cavity plate confront the permanent magnet. In a further preferred form, the means for rotating the cavity ring is a rotating handle which is manually operated. In yet another preferred form, the magnet material for the permanent magnets is of the neodymium (Ne-Fe-B) type, the samarium cobalt (Sm-Co) type, or the alnico (Al-Ni-Co) type. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of an injection mold for an optical disc substrate, which injection mold makes use of a first embodiment of a stamper plate attaching/detaching device of an injection mold for an optical disc substrate in accordance with the present invention; FIG. 2A and 2B are explanatory view of the operation of the stamper plate attaching/detaching device of the injection mold for the optical disc substrate shown in FIG. 1; FIG. 3 is an explanatory view of the operation of a second embodiment of the stamper plate attaching/detaching device of the injection mold for the optical disc substrate in accordance with the present invention; and FIG. 4 is a cross-sectional view of the structure of an injection mold for an optical disc substrate which employs a conventional cavity ring. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in more detail with reference primarily to the accompanying drawings. FIG. 1 is a cross-sectional view of an essential part of a mold which employs the stamper plate attaching/detaching device of an injection mold for an optical disc substrate in accordance with the present invention. Shown on the left side of the drawing is a state wherein a cavity ring has been installed on a cavity side disc cavity plate and the stamper plate has been attached. Shown on the right side of the drawing is a state wherein the cavity ring has been attached to a movable disc cavity plate and the mold is open for the stamper plate to be attached or detached. A cylindrical stamper plate holding means 2 is equipped with a body which is inserted in the central hole in a stamper plate 26 and a flange 3 which is larger than the central hole in the stamper plate 26. A cavity side disc cavity plate 15 is fixed to a cavity side base plate 24. A sprue bush 8 is provided at the center of the aforesaid components. Resin is injected into the cavity through an injection hole 9 for melted resin at the center of the sprue bush 8. A movable disc cavity plate 16 is provided on a movable base plate 25; a punch 12 for punching the central hole at the center of a disc substrate is inserted in the central hole of the movable disc cavity plate 25. A cavity ring 17 forms, in cooperation with the cavity side disc cavity plate 15 and the movable disc cavity 16, a cavity hollow for injection molding when the mold is clamped. The cavity ring 17 also serves to press the outer circumferential rim of the stamper plate against the cavity side cavity plate 15. The cavity ring 17 is provided with a handle 30 for moving the cavity ring 17 between a first angle position and a second angle position in relation to the cavity side disc cavity plate 15 and the movable disc cavity 16. The cavity ring 17 has an inside diameter 28 which is smaller than an outside diameter 27 of the stamper plate 26; it is rotatably fitted onto an outer circumferential stepped section 29 of the movable disc cavity plate 16. FIG. 2A and 2B illustrate the relationship among the hole provided in the cavity ring 17, the cavity side disc cavity plate 15, the movable disc cavity plate 16, and the permanent magnets provided thereon. The angular positional relationship given in FIG. 2A corresponds to the state shown on the left side in FIG.1; the angular positional relationship given in FIG. 2B corresponds to the state shown on the right side in FIG. 1. Six blind holes 18 are provided along the same radial line at equal angles from the outer circumferential rim of the cavity side disc cavity plate 15 and six blind holes 19 are provided along the same radial line from the outer circumferential rim of the movable disc cavity plate 16 in such a manner that they face against the blind holes 18, the relative angles thereof being shifted by 15 degrees. Likewise, the cavity ring 17 is also provided with six through holes 20 laid out along the same radial line. The disc cavity plates 15 and 16 respectively have six neodymium (Ne-Fe-B) rod-shaped permanent magnets 22 and 23 inserted in bushes 21 made of a nonmagnetic material such as ferrite-based stainless in the six blind holes 18 and 19, respectively, on the same radial line mentioned above. Shown at left in FIG. 1 is a state wherein the cavity ring 17 has been mounted on the cavity side disc cavity plate 15; shown at right in the same drawing is a state wherein the cavity ring 17 has been moved to the movable disc cavity plate 16. An optical disc substrate is injection-molded while the outer circumferential rim of the stamper plate 26 is being pressed by the cavity ring 17 so that it is brought in close contact with the surface of the cavity side disc cavity plate 15. For this reason, the cavity ring 17 must be mounted and fixed onto the cavity side disc cavity plate 15. The stamper plate 26 cannot be removed with the cavity ring 17 mounted and fixed onto the cavity side disc cavity plate 15. Hence, the cavity ring 17 must be moved onto the movable disc cavity plate 16 first, then the mold is opened to take out the stamper plate 26. FIG. 2A and FIG.2B show the positional relationship between the respective permanent magnets and the holes when the cavity ring 17 is attached to the cavity side disc cavity plate 15 and the movable disc cavity plate 16, respectively. As shown in FIG. 2A, when the cavity ring 17 is attached to the cavity side disc cavity plate 15, the rim surface portion except the through hole 20 of the cavity ring 17 overlaps the permanent magnet 22 of the cavity side disc cavity plate 15; the through hole 20 overlaps the permanent magnet 23 of the movable disc cavity plate 16. The through hole 20 is manually moved by the handle 30 of the cavity ring 17 to an angular position where it overlaps the permanent magnet 23 of the movable disc cavity plate 16. The attractive force given by the permanent magnet 22 of the cavity side disc cavity plate 15 causes the cavity ring 17 to be attracted and fixed to an outer circumferential stepped surface 31 of the cavity side disc cavity plate 15. At this time, the position of the permanent magnet 23 of the movable disc cavity plate 16 coincides with the position of the through hole of the cavity ring 17; therefore, the attractive force is not applied to the cavity ring 17. Shown in FIG. 2B is the state wherein the cavity ring 17 has been moved from the cavity side disc cavity plate 15 to the movable disc cavity plate 16. More specifically, with the mold incompletely clamped with a gap of 0.5 mm or less still left, the cavity ring 17 is rotated manually in the opposite direction by the handle 30 so that the rim surface portion of the cavity ring 17 except the through hole 20 overlaps the permanent magnet 23 of the movable disc cavity plate 16 and the through hole 20 coincides with the position of the permanent magnet 22 of the cavity side disc cavity plate 15. This causes the cavity ring 17 to be attracted to the movable disc cavity plate 16 by the attractive force given by the permanent magnet 23 of the movable disc cavity plate 16. The attractive force of the permanent magnet 22 of the cavity side disc cavity plate 15 does not work on the surface of the cavity ring 17. The stoppers at both ends of rotational range of the cavity ring 17, a detent, and the like are provided according to a publicly known method although they are not shown. Shown in FIG. 2A is the positional relationship established when the cavity ring 17 is in the first angle position. Under the positional relationship, an attractive force is generated between the permanent magnet 22 of the cavity side disc cavity plate 15 and the cavity ring 17, whereas no attractive force is generated between the permanent magnet 23 of the movable disc cavity plate 16 and the cavity ring 17. Shown in FIG. 2B is the positional relationship established when the cavity ring 17 is in the second angle position. Under the positional relationship, no attractive force is generated between the permanent magnet 22 of the cavity side disc cavity plate 15 and the cavity ring 17, whereas the attractive force is generated between the permanent magnet 23 of the movable disc cavity plate 16 and the cavity ring 17. FIG. 3 is an explanatory diagram for illustrating the structure and operation of a second embodiment of the stamper plate attaching/detaching device of the injection mold for the optical disc substrate in accordance with the present invention. The basic structure of the mold to which the second embodiment is to be applied is identical to that described with reference to FIG. 1 and FIG. 4. Six permanent magnets 35, which have magnetic poles oriented in the vertical direction, are buried in the through holes in a cavity ring 32 via bushes 34 composed of a nonmagnetic material including a feeble or diamagnetic material. The cavity side disc cavity plate 36 and the movable disc cavity plate 37 are respectively provided with blind holes 38 and 39 which are shifted in phase against each other by 15 degrees. The procedure for attaching and detaching the stamper plate of the injection mold for the optical disc substrate is the same as that for the first embodiment. FIG. 3 shows a state wherein the cavity ring 32 is in the second angular position in relation to the cavity side disc cavity plate 36 and the movable disc cavity plate 37. In this state, no attractive force is generated between the cavity side disc cavity plate 36 and the permanent magnet 35, whereas the attractive force is generated between the movable disc cavity plate 37 and the permanent magnet 35. When the cavity ring 32 is in the first angular position (not shown) in relation to the cavity side disc cavity plate 36 and the movable disc cavity plate 37, the attractive force is generated between the cavity side disc cavity plate 36 and the permanent magnet 35, whereas no attractive force is generated between the movable disc cavity plate 37 and the permanent magnet 35. The injection molds for the optical disc substrates which employ the embodiments described above have the following advantages. Firstly, the stamper plate does not come off as in the case of a vacuum suction type device. Secondly, according to the present invention, there is no need to change the material of the disc cavity plates or to add a change to the conventional heat transfer structure since the magnets are not buried in the surfaces of the disc cavity plates which correspond to the signal bit area of the stamper plate. This makes it possible to use the basic structure of a field-proven, reliable mold for the disc substrate. Thirdly, since no mounting bolts are involved for attaching the cavity ring to the disc cavity plates, the stamper plate can be attached or detached quickly, securely and safely with the mold installed to the injection molding machine. The present invention can be embodied in other specific forms without departing from the essential characteristics thereof. For instance, the permanent magnets may be buried in both cavity plate and cavity ring so that the attractive force is generated when the surfaces of opposite poles face against each other and a repulsive force is generated when the surfaces of the same pole face against each other.

Claims: A stamper plate attaching/detaching device of an injection mold for an optical disc substrate for supporting the stamper plate in a cavity which is formed by a cavity side disc cavity plate (15), a movable disc cavity plate (16), and a cavity ring (17); said stamper plate attaching/detaching device comprising: a section for holding the outer circumferential rim of said stamper plate by means of the inner circumferential rim of said cavity ring (17), which inner circumferential rim is made larger than the outer diameter of said stamper plate; characterized by : a plurality of pairs of permanent magnets (22, 23) which have magnetic poles oriented in parallel to the central line and which are buried, with phase differences, along the same radial line of said cavity side disc cavity plate (15) and said movable disc cavity plate (16); rotating means (30) for giving said cavity ring (17) a relative rotation between a first angle position and a second angle position in relation to said cavity side disc cavity plate and said movable disc cavity plate; and an attractive force generating structure designed to cause an attractive force to be generated between the permanent magnets of said cavity side disc cavity plate and said cavity ring but cause no attractive force to be generated between the permanent magnets of said movable disc cavity plate and said cavity ring when said cavity ring is in the first angle position; and to cause no attractive force to be generated between the permanent magnets of said cavity side disc cavity plate and the cavity ring but to cause the attractive force to be generated between the permanent magnets of said movable disc cavity plate and said cavity ring when said cavity ring is in the second angle position. A stamper plate attaching/detaching device of the injection mold for the optical disc substrate according to Claim 1, further comprising stamper plate central hole holding means (2) which holds the central hole in said stamper plate, which is pressed by said cavity ring against either disc cavity, against said disc cavity. A stamper plate attaching/detaching device of the injection mold for the optical disc substrate according to Claim 1, wherein said cavity ring (17) is composed of a ferromagnetic material and it is provided with a hole (20) which prevents an attractive force from being generated when it confronts any one pair of said permanent magnets (22, 23). A stamper plate attaching/detaching device of the injection mold for the optical disc substrate according to Claim 1, wherein said means for rotating the cavity ring is a rotating handle (30) which is manually operated. A stamper plate attaching/detaching device of the injection mold for the optical disc substrate according to Claim 1, wherein the magnet material for the permanent magnets (22, 23) is of a neodymium (Ne-Fe-B) type, a samarium cobalt (Sm-Co) type, or an alnico (Al-Ni-Co) type. A stamper plate attaching/detaching device of the injection mold for the optical disc substrate for supporting said stamper plate in the cavity which is formed by a cavity side disc cavity plate (36), a movable disc cavity plate (37), and a cavity ring (32), said stamper plate attaching/detaching device comprising: a section for holding the outer circumferential rim of the stamper plate by means of the inner circumferential rim of said cavity ring, which inner circumferential rim is made larger than the outer diameter of said stamper plate; characterized by : a plurality of pairs of permanent magnets (35) which have magnetic poles oriented in parallel to the central line and which are buried along the same radial line of said cavity ring (32); and rotating means for giving said cavity ring a relative rotation between a first angle position and a second angle position in relation to said cavity side disc cavity plate and said movable disc cavity plate; wherein said cavity side disc cavity plate and the movable disc cavity plate are provided with a structure designed to cause an attractive force to be generated between said cavity side disc cavity plate and said permanent magnets but to cause no attractive force to be produced between said movable disc cavity plate and the permanent magnets when said cavity ring is in said first angle position; and to cause no attractive force to be generated between said cavity side disc cavity plate and said permanent magnets but to cause the attractive force to be generated between said movable disc cavity plate and the permanent magnets when said cavity ring is in said second angle position. A stamper plate attaching/detaching device of the injection mold for the optical disc substrate according to Claim 6, further comprising a stamper plate central hole holding means (2) which holds the central hole in said stamper plate, which is pressed by said cavity ring against either disc cavity, against said disc cavity. A stamper plate attaching/detaching device of the injection mold for the optical disc substrate according to Claim 6, each of said cavity side disc cavity plate (36) and said movable disc cavity plate (37) further comprising a ferromagnetic section which generates an attractive force when it confronts said permanent magnets (35) of said cavity ring (32) of said cavity side disc cavity plate (36) and said movable disc cavity plate (37) and a hole (38, 39) or a nonmagnetic section which prevents the attractive force from being generated when said cavity side disc cavity plate and said movable disc cavity plate confront said permanent magnet. A stamper plate attaching/detaching device of the injection mold for the optical disc substrate according to Claim 6, wherein the means for rotating said cavity ring is a rotating handle (30) which is manually operated. A stamper plate attaching/detaching device of the injection mold for the optical disc substrate according to Claim 6, wherein the magnet material for the permanent magnets (35) is of the neodymium (Ne-Fe-B) type, the samarium cobalt (Sm-Co) type, or the alnico (Al-Ni-Co) type.

Inventors: TAKAHASHI MITSUO, YASUDA KATSUYUKI, TAKAHASHI, MITSUO, YASUDA, KATSUYUKI
Applicants: SEIKO GIKEN KK, SEIKOH GIKEN CO., LTD.
Application year: 1996
Main classification: B29D17
IPC codes: B29C45, B29D17, B29C33, G11B7, B29L17