Daicel Chemical Industries, Ltd.

Yoshio Onizawa

Mitsuru Yamashita

Akihiro Izuka

Norio Okabayashi

Shuzo Iyoshi

Yasushi Hiramatsu

Shinji Tokuhara

A recording medium for optical high-density data storage and retrievel, comprising a flat transparent plastic substrate characterized by the following relationship between the refractive index (n.sub.z) in the direction perpendicular to the flat surfaces of the transparent substrate, and the refractive indexes (n.sub.x) and (n.sub.y) in the direction parallel to the flat surfaces of the transparent substrate:

Process for producing plastic substrate for optical disks

A recording medium for optical high-density data storage and retrieval, comprising a flat transparent plastic substrate characterized by the following relationship between the refractive index (n.sub.z) in the direction perpendicular to the flat surfaces of the transparent substrate, and the refractive indexes (n.sub.x) and (n.sub.y) in the direction parallel to the flat surfaces of the transparent substrate:

Plastic substrate for optical disk and process for producing the same

A disk substrate for a high-density information recording medium made by injection molding from polycarbonate is disclosed having an average molecular weight from 14,000 to 22,000 and having a thickness from 1.0 to 2.0 mm and a circular recording area of between two concentric circles having a radii of 40 to 170 mm, respectively. The birefringence of the substrate within the recording area is less than 25 mm and is uniformly distributed.

Large diameter optical disk substrate made of polycarbonate

An injection mold for injection molding a plastic substrate for high-density information recording medium. An annular sliding member is axially slidably supported in the circumference of a disk-shaped cavity of a mold half. The annular sliding member is biased toward the surface of a disk-shaped cavity provided in another mold half so as to form the molding cavity and maintain contact with said surface during molding of said plastic substrate.

Injection mold for an optical disk

A metal mold to be used for injection-molding of a flat plastic disc. The flat, annular mold cavity has a substantially continuously increasing axial thickness away from the injection gate. The gate can be in the center or on the outer periphery of the mold. A split mold type is disclosed wherein at least one of the inner opposed surfaces tapers away from an imaginary flat center plane. Alternatively, a stamper having a tapering thickness can be used in a mold having parallel opposed inner surfaces.

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Hyogo, Japan

Yoshio Onizawa