For computer's storing units and so forth, disc shaped record mediums such as reproduction-only optical discs, magneto-optical discs, and phase-change type optical discs (hereinafter, these record mediums are referred to as optical discs) to and from which information signals are written and read using light have been becoming common. A conventional optical disc is composed of a disc substrate made of resin. For example, a compact disc is composed of a disc substrate having a thickness of 1.2 mm and on which signals were recorded as a pattern of concave and convex portions (pits). A DVD is composed of two disc substrates having a thickness of 0.6 mm which are adhered to one another.
A disc substrate is produced by injection-molding a heated and molten resin material. For example, a pellet-shaped resin material is sufficiently dried by a drying unit. After moisture of the resin material is sufficiently removed, it is placed in an injection molding unit. Thereafter, the resin material is supplied to a cylinder of the injection molding unit. The resin material is heated and molten by the cylinder. The heated and molten resin material is injected into a closed space formed in a disc substrate molding portion (hereinafter, the closed space is referred to as a cavity). In the cavity, a stamper is disposed. By solidifying the resin material, a concave and convex pattern of the stamper is transferred to the resin material. As a result, a disc substrate is produced.
A molding die for a disc substrate is composed of a fixed side mirror which molds one main surface of a substrate, a moving side mirror which molds the other main surface of the substrate, and an outer periphery ring which molds an outer periphery portion of the substrate. FIG. 1 is a sectional view showing a conventional die. The die is composed of a fixed side die member 101 and a moving side die member 104.
The fixed side die member 101 is disposed in an injection molding unit main body (not shown) in such a manner that the fixed side die member 101 is fixed thereto. A sprue bush 102 which has a path for a molten resin material is fitted into the fixed side die member 101. In the sprue bush 102, a resin injection opening 102a is formed. The resin injection opening 102a is positioned at the center of a molding cavity 103. The resin injection opening 102a injects a molten synthesized resin material such as polycarbonate resin supplied from the injection unit side into the cavity 103.
The moving side die member 104 is disposed in such a manner that it can moved to and from the fixed side die member 101 (in the left and right directions in FIG. 1). On the moving side die member 104, a molding surface 104b of a moving side mirror 104d is disposed in opposition to a stamper 101b which forms a molding surface of a fixed side mirror 101d. A center punch which punches a center opening into the molded substrate and protruding members 106a and 106b which are positioned on an outer periphery side and an inner periphery side of the center punch 105 and remove the molded substrate from the cavity 103 are disposed in the moving side die member 104.
An outer periphery ring 107 is formed in a ring shape. The outer periphery ring 107 is slidably disposed in a groove formed by an interlock ring 108 on an outer periphery side of the moving side mirror 104d. When the fixed side die member 101 is contacted with the moving side die member 104, the cavity 103 is formed between the stamper 101b, which is the molding surface of the fixed side mirror 101d, the molding surface 104b of the moving side mirror 104d, and the outer periphery ring 107. Portions 101c, 102b, 104c, and 105a are temperature adjusting circuits formed in the fixed side mirror 101d, the sprue bush 102, the moving side mirror 104d, and the center punch 105, respectively. In the interlock ring 108, a plurality of gas relief openings 109 which relieve gas produced by the resin material filled in the cavity 103 are formed.
When a disc substrate is produced by such a die using the injection molding method, since small gaps are formed in engagement portions 110a and 110b of the outer periphery surface of the moving side mirror 104d and the inner periphery surface of the outer periphery ring 107 (hereinafter the gaps are referred to as clearances), an outer periphery portion of the produced disc substrate is burred in a direction perpendicular to the substrate. In the case of a DVD, the burring prevents two disc substrates from being adhered. In addition, when a label is printed on a disc, the printing block may be caught by the burring. Although the engagement portions 110a and 110b represent the same ring-shaped engagement portion, for an easy explanation, they have been designated by different reference numerals.
As with the example shown in FIG. 1, when the outer periphery ring 107 is structured as a sliding ring, in order to suppress the outer periphery of the disc substrate from being burred, it is necessary to decrease the clearance formed between the inner periphery surface of the outer periphery ring 107 and the outer periphery surface of the moving side mirror 104d. However, when the clearance is small, there is a possibility of which the outer periphery ring 107 cannot be smoothly slid. Thus, even if the clearance cannot be completely removed, it is necessary to suppress the outer periphery portion of the disc substrate from being burred as much as possible.
In order to suppress the outer periphery portion of the disc substrate from being burred, it is necessary to cause the sizes of the clearances of the ring-shaped engagement portions represented by the engagement portions 110a and 110b to become equal. In an example shown in FIG. 2, the size of a clearance of an engagement portion 110a is different from the size of a clearance of an engagement portion 110b. Thus, the sizes of the clearances are not equal. In this case, the larger clearance the engagement portion has, the more burred the outer periphery portion of the disc substrate is. Consequently, the forgoing problems become more remarkable.
In order to suppress the outer periphery portion of the disc substrate from being burred, a precise sliding structure may be accomplished by a bearing or the like interposed between an outer periphery ring 107 and a moving side mirror 104d so as to cause the sizes of the clearances to become equal on the entire periphery of the outer periphery ring 107.
However, when a bearing is disposed on the clearance side of the outer periphery ring, an operating trouble may take place in the bearing in such a manner that gas produced by resin filled in the cavity is solidified and adhered to a metal portion of the bearing and thereby the bearing does not operate. When the operating trouble takes place in the bearing, the dead weight of the moving side cast member causes engagement clearances to deviate. As a result, since the outer periphery of the outer periphery ring and the inner periphery of the interlock ring get worn, the maintenance period cannot be prolonged as an inconvenient problem. In order to prevent a solidified substance of gas from being adhered to the bearing, a structure in which the bearing is disposed outside the outer periphery ring may be considered. In this case, however, the bearing tends to be collapsed by the pressure of the injected resin material. In other words, the structure in which the bearing is interposed does not provide a preferable solution for suppressing the outer periphery portion of the disc substrate from being burred.
Therefore, an object of the present invention is to provide a molding die for a disc substrate which allows the clearances of the engagement portions formed by the outer periphery ring and the moving side mirror to be equal and the outer periphery portion of the disc substrate to be suppressed from being burred.