Stamper plate mounting device of injection mold for optical disc substrate

Abstract: A stamper plate mounting device for an injection mold for an optical disc substrate has a simple structure and permits easy attachment and detachment of a stamper plate without impairing the basic functions of a molding machine. A stamper plate is detachably supported in a cavity hollow which is formed between a cavity side disc cavity plate and a movable disc cavity plate of the injection mold for the optical disc substrate. A central hole is provided at the center of the disc cavity. The stamper plate mounting device has: a means for holding the inner circumference of a stamper plate which has, on one end thereof, a flange for holding the inner circumference of the stamper plate and which is precisely fitted in the central hole from the other end thereof; and a permanent magnet which is buried in the bottom surface of the central hole or in the other end of the means for holding the inner circumference of the stamper plate and which has a magnetic pole in the axial direction.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stamper plate mounting device of an injection mold for an optical disc substrate for a CD-ROM or the like. 2. Description of the Related Art Some of conventional stamper plate mounting devices of injection molds for optical disc substrates are disadvantageous in that they require complicated operation or they do not permit easy removal of stamper plates thereof; some of them have such structures that they may impair the basic functions of metal molds. There has been known an injection mold for optical disc substrates which is designed to move a stamper plate holding bush by a camshaft (refer to, for example, Japanese Patent Laid-Open No. 2-295726). FIG. 3 is a cross-sectional view illustrative of a conventional injection mold for optical disc substrates which is in the middle of an injection molding process, wherein the aforesaid camshaft is used to move the stamper plate holding bush. FIG. 4 is a schematic cross-sectional view illustrating the conventional injection mold wherein the stamper plate holding bush has been moved. In both drawings, the cavity side parts are placed, facing up; however, the metal mold may be placed sideways so that the central axis of the cavity may be horizontal, or it may also be placed upside down from that shown in the drawings. A retainer plate 6 is provided with a cavity side disc cavity plate 4. Provided at the center of the cavity side disc cavity plate 4 is a hole 5 in which a stamper plate holding bush 2 is inserted. The bottom end of the stamper plate holding bush 2 is provided with a flange 3 which has a larger outside diameter than the central hole of the stamper plate 1. The top end of the stamper plate holding bush 2 is' provided with notch 9. The retainer plate 6 rotatably supports two engaging rods 7 which engage with the aforesaid notch 9 of the bush 2. The two engaging rods 7, 7 respectively have planes 8, 8 of semicircular cross sections and they are connected so that they rotate in synchronization. A sprue bush 10 is inserted in the central hole in the stamper plate holding bush 2; it has an injection hole 11 for injecting melted resin at the center thereof. The cavity side disc cavity plate 4 is equipped with an annular cooling water channel 12 which is in contact with the retainer plate 6. Fixed on a movable retainer plate 19 is a movable disc cavity plate 17. A punch 18 for punching a central hole in a disc substrate is supported at the center of the movable disc cavity plate 17 in such a manner that it can move up and down in the drawing. At the beginning of injection molding, the stamper plate 1 is fitted into the stamper plate holding bush 2 and the outer circumference of the hole of the stamper plate is held by the flange 3 of the bush 2. During the injection molding, the bush 2 is retained by engagement with the planes 8, 8 of semicircular cross sections of the two engaging rods 7, 7 as shown in FIG. 3. Referring to FIG. 4, the procedure for detaching the stamper plate 1 will be described. The mold is opened and a disc substrate molding 20 and the two engaging rods 7 are turned in the directions of the arrows as shown in FIG. 4. This releases the planes 8 of the engaging rods 7 from the notch 9 of the bush 2 for mounting the stamper plate, thus pushing the top surface thereof in the drawing. This in turn pushes the bush 2 for mounting the stamper plate out of the central hole 5 of the cavity side disc cavity plate 4 as illustrated, enabling an operator to manually remove the stamper plate 1 while holding the stamper plate holding bush 2. The example of the conventional stamper plate mounting device described above is designed to mechanically attach and detach the stamper plate 1 to and from the bush 2 for mounting the stamper plate, thus ensuring the attachment and detachment of the stamper plate. This design, however, requires the use of the two rods 7, 7 and a device for driving them, thus posing the following problem. The use of the two rods and the provision of the driving device therefor in relation to a metal mold adversely affect the efforts made in achieving a smaller, lighter metal mold. Moreover, providing the rods and the driving device therefor in relation to the metal mold unavoidably adds restrictions to the design of the cooling water channel and it also requires a certain level of skill for attaching and detaching the stamper plate. 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 C55 to 60 and further 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 technique including 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 mounting device of an injection mold for an optical disc substrate, which has a simple structure and which allows a stamper plate to be easily attached or detached without impairing the fundamental functions of a molding machine. To this end, according to the present invention, there is provided a stamper plate mounting device of an injection mold for an optical disc substrate; wherein a stamper plate is detachably supported in a cavity hollow which is formed between a cavity side disc cavity plate and a movable disc cavity plate of the injection mold for the optical disc substrate; comprising: a central hole provided at the center of a disc cavity; means for holding the inner circumference of the stamper plate, which means has a flange for holding the inner circumference of the stamper plate on one end thereof and which is precisely fitted in the central hole from the other end thereof; and a permanent magnet which is buried in the bottom surface of the central hole or in the other end of the means for holding the inner circumference of the stamper plate and which has a magnetic pole in the axial direction; wherein at least a surface confronting the permanent magnet is composed of a ferromagnetic material, so that the stamper plate is attached to the cavity by the attraction of the permanent magnet when the means for holding the inner circumference of the stamper plate is inserted in the central hole. In a preferred form, the permanent magnet mentioned above is cylindrical and it is embedded in the other end surface of the means for holding the inner circumference of the stamper plate, so that it works on a bottom surface of the central hole of the aforesaid cavity plate. In another preferred form, the permanent magnet can be buried in the bottom surface of the central hole 40 of the cavity plate so as to work on the other end surface of a means 45 for holding the inner circumference of the stamper plate. In a further preferred form, the magnet material for the permanent magnet may be of a neodymium (Ne-Fe-B) type, a samarium cobalt (Sm-Co) type, or alnico (Al-Ni-Co) type. In another preferred form, the body of the means for holding the inner circumference of the stamper plate has an outside diameter surface which precisely fits into the central hole of the cavity plate; it may be provided with a flange which is larger than the aforesaid outside diameter surface. The means for holding the inner circumference of the stamper plate is constituted by an inner circumference bush and an outer circumference bush. One end of the inner circumference bush is provided with an outside diameter surface, which precisely fits in the central hole of the stamper plate, and a flange which is larger than the central hole of the stamper plate; the outer circumference is provided with an external thread. The outer circumference bush has an internal thread, which engages with the external thread and which is formed on the inner circumferential surface, and also the outside diameter surface which can be inserted in or withdrawn from the central hole of the cavity side disc cavity plate. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a first embodiment of a stamper plate mounting device of an injection mold for an optical disc substrate in accordance with the present invention; FIG. 2 is a cross-sectional view of a second embodiment of a stamper plate mounting device of an injection mold for the optical disc substrate in accordance with the present invention; FIG. 3 is a schematic side cross-sectional view illustrative of resin being charged into the injection mold for the optical disc substrate, which mold employs the stamper plate attaching and detaching device of an injection mold employing a stamper plate mounting device of a conventional optical disc substrate injection mold; and FIG. 4 is a cross-sectional view for illustrating how the conventional stamper plate of the optical disc substrate injection mold shown in FIG. 3 is removed. DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a cross-sectional view of a first embodiment of a stamper plate mounting device of an injection mold for an optical disc substrate in accordance with the present invention. To simplify the description, a state wherein the stamper plate has been attached is shown on the left side and whereas a state wherein the stamper plate has not yet been attached is shown on the right side. A hollow in the cavity is formed between a cavity side disc cavity plate 28 and a movable disc cavity plate (not shown) and melted resin is injected via the sprue bush 10, thereby injection-molding an optical disc substrate. The cavity side disc cavity plate 28 is provided on the retainer plate 6 and the hole 29 is provided at the center of the cavity side disc cavity plate 28. At least the bottom surface 30 and the circumferential surface of the hole must be composed of a ferromagnetic material. A cylindrical member 46, which supports the sprue bush 10 onto the cavity side disc cavity plate 28, is composed of a nonmagnetic material or a weak magnetic material, so that it exhibits higher magnetoresistance than the ferromagnetic material. The flange 25, which is larger than a central hole 23 of a stamper plate 22, is provided on one end of the means 21 for holding the inner circumference of the stamper plate. The outside diameter surface 24 of the holding means is precisely fitted into the central hole 23 of the stamper plate 22 and the central hole 29 of the cavity side disc cavity plate 28. The other end of the means 21 for holding the inner circumference of the stamper plate is equipped with the cylindrical permanent magnet 27 which generates a magnetic flux applied to the bottom of the disc cavity. The permanent magnet 27 is, for example, an annular neodymium (Ne-Fe-B) permanent magnet; in the drawing, the north pole is on the upper side and the south pole is on the lower side in the drawing. The permanent magnet 27 is buried in the means 21 for holding the inner circumference of the stamper plate via a ferrite-based stainless bush, which is composed of a nonmagnetic material rather than a ferromagnetic material. As the permanent magnet, a samarium cobalt (Sm-Co) type or alnico (Al-Ni-Co) type permanent magnet may be used in place of the neodymium type. To fit the stamper plate 22 in the central hole 29 of the disc cavity plate 28, the stamper plate 22 is first fitted onto the stamper plate mounting bush 21, then fitted in the central hole 29 of the disc cavity plate 28. This creates a magnetic circuit shown on the left side in FIG. 1 (the north pole of the permanent magnet 27 → gap → the bottom surface 30 of the cavity side disc cavity plate 28 → the outer wall circumference of the hole of the cavity side disc cavity plate 28 → the outer portion of the means 21 for holding the inner circumference of the stamper plate of a nonmagnetic member 26 which has been inserted in the means 21 for holding the inner circumference of the stamper plate → the south pole of the permanent magnet 27). The magnetic circuit generates attracting force between the means 21 for holding the inner circumference of the stamper plate and the cavity side disc cavity plate 28. To detach the means 21 for holding the inner circumference of the stamper plate, a means (not shown) for generating a magnetic force which is stronger than that of the permanent magnet 27 is actuated to apply the magnetic force to the means 21 for holding the inner circumference of the stamper plate so as to cancel the magnetic attraction between the means 21 for holding the inner circumference of the stamper plate and the bottom surface 30 of the cavity side disc cavity plate 28, thereby detaching the means 21. As an alternative, instead of using the magnetic force generating means, compressed air may be supplied between the means 21 for holding the inner circumference of the stamper plate and the cavity side disc cavity plate 28 in order to push the means 21 out. FIG. 2 is a cross-sectional view of a second embodiment of the stamper plate mounting device of the injection mold for the optical disc substrate in accordance with the present invention. The left half of the drawing shows a state wherein the stamper plate has been mounted. The hole 40 is provided at the center of the disc cavity plate 39. An annular permanent magnet 44 is disposed on a bottom surface 42 of the hole 40. The annular, cylindrical permanent magnet 44 made of neodymium (Ne-Fe-B) is buried in the bottom of the disc cavity plate 39 via a bush 43 composed of a nonmagnetic material. As the permanent magnet, a samarium cobalt (Sm-Co) type or alnico (Al-Ni-Co) type permanent magnet may be used in place of the neodymium type. The means 45 for holding the inner circumference of the stamper plate is constituted by an inner circumference bush 33 and an outer circumference bush 37. One end of the inner circumference bush 33 is provided with an outside diameter surface 34, which precisely fits in the central hole 31 of the stamper plate 32, and the flange 35 which is larger than the central hole 31 of the stamper plate 32; the outer circumference is provided with the external thread 36. The outer circumference bush 37 is made of a ferromagnetic material; it has an internal thread 38, which engages with the external thread 36 and which is formed on the inner circumferential surface, and also the outside diameter surface 41 which can be inserted in or withdrawn from the central hole 40 of the cavity side disc cavity plate 39. The procedure for fitting the stamper plate 32 of this embodiment in the central hole 40 of the cavity side disc cavity plate 39 is as follows: firstly, the stamper plate 32 is fitted onto the outside diameter surface 34 of the inner circumference bush 33; secondly, the inner circumference bush 33 is attached to the outer circumference bush 37, then they are inserted in the central hole 40 of the cavity side disc cavity plate 39. This creates a magnetic circuit as shown on the left side in FIG. 2 (the north pole of the permanent magnet 44 → the cavity side disc cavity plate 39 → the outer circumference bush 37 → the south pole of the permanent magnet 44). This magnetic circuit generates attracting force between the means 45 for holding the inner circumference of the stamper plate and the cavity side disc cavity plate 29. The procedure for detaching the means 45 for holding the inner circumference of the stamper plate is the same as that for the first embodiment. The stamper plate mounting device of the injection mold for the optical disc in accordance with the present invention has the following advantages over the conventional stamper plate mounting device. Firstly, the means for holding the inner circumference of the stamper plate, to which a stamper plate has been attached, is securely held by the attraction given by the permanent magnet; therefore, the stamper plate does not come off as in the case of the example of the conventional vacuum suction type device. Secondly, according to the present invention, there is no need to change the material of the disc cavity plate or to modify the heat transfer structure thereof as in the conventional device since the magnet is not buried in the surface of the disc cavity plate which corresponds to the signal bit area of the stamper plate. Thirdly, unlike the conventional device shown in FIG. 3 and FIG. 4, the device according to the present invention does not require any mechanical driving system, thus making it possible to achieve a smaller, lighter metal mold and also to reduce the number of components. In addition, a higher degree of freedom in designing makes it possible to create an injection mold for optical disc substrates which provides better performance. The present invention can be embodied in other specific forms without departing from the essential characteristics thereof. In the embodiments described above, the stamper plate is mounted onto the cavity side disc cavity plate; however, the stamper plate may alternatively be mounted onto the movable disc cavity plate.

Claims: A stamper plate mounting device of an injection mold for an optical disc substrate; wherein a stamper plate (22,32) is detachably supported in a cavity hollow which is formed between a cavity side disc cavity plate (28) and a movable disc cavity plate of the injection mold for the optical disc substrate; comprising: a central hole (29,40) provided at the center of a disc cavity; means (21,45) for holding the inner circumference of the stamper plate, which means has a flange for holding the inner circumference of the stamper plate on one end thereof and which is precisely fitted in said central hole from the other end thereof; and characterized by a permanent magnet (27,44) which is buried in the bottom surface (30) of said central hole or in the end of said means (21,45) for holding the inner circumference of the stamper plate facing said bottom surface (30) of said central hole (29, 40) and which has a magnetic pole in the axial direction; wherein at least a surface confronting said permanent magnet is composed of a ferromagnetic material, so that the stamper plate is attached to the cavity by the attraction given by the permanent magnet when said means for holding the inner circumference of the stamper plate is inserted in said central hole. A stamper plate mounting device of the injection mold for the optical disc substrate according to Claim 1, wherein said permanent magnet (45) is cylindrical and it is buried in the surface on the end of said means for holding the inner circumference of said stamper plate facing said bottom surface (30) of said central hole (40), and it is configured so as to work on the bottom surface of the central hole in said cavity plate. A stamper plate mounting device of the injection mold for the optical disc substrate according to Claim 1, wherein said permanent magnet (27) is buried in the bottom surface of a central hole (29) in the cavity plate and it is configured so as to work on the surface of the other end of said means for holding the inner circumference of said stamper plate. A stamper plate mounting device of the injection mold for the optical disc substrate according to Claim 1, wherein the magnet material for said permanent magnet (27,44) is of a neodymium (Ne-Fe-B) type, a samarium cobalt (Sm-Co) type, or alnico (Al-Ni-Co) type. A stamper plate mounting device of the injection mold for the optical disc substrate according to Claim 1, wherein the body of said means (21) for holding the inner circumference of the stamper plate has an outside diameter surface which precisely fits in the central hole (29) in said cavity plate and it is equipped with a flange (25) which has a larger diameter than said outside diameter surface. A stamper plate mounting device of the injection mold for the optical disc substrate according to Claim 1, wherein said means (45) for holding the inner circumference of the stamper plate is constituted by: an inner circumference bush (33) which is provided, on one end thereof, with an outside diameter surface, which precisely fits in the central hole of the stamper plate, a flange (35) which has a larger diameter than the central hole of the stamper plate, and an external thread on the other end thereof; and outer circumference bush (37) which has an internal thread, which engages with the external thread and which is formed on the inner circumferential surface, and which also has an outside diameter surface which can be fitted in or withdrawn from the central hole of the cavity side disc cavity plate.

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