Method of producing glass blank for substrate of information recording medium, substrate for information recording medium, and information recording medium; and manufacturing apparatus for glass blank for substrate of information recording medium

1. A method of manufacturing a glass blank for a substrate for an information recording medium, the method including steps of:

cutting out a gob of a glass material from a molten glass material by cutting the molten glass material discharged from a glass material outlet at a predetermined timing, so as to separate the gob of the glass material from the molten glass material;

causing the separated gob of the glass material to fall downwardly so that the separated gob falls into a space between a pair of molds of a press unit;

catching the separated gob of the glass material between the pair of molds, and pinching and pressing the separated gob of the glass material by the pair of molds, the molds' surfaces facing to each other being press surfaces which are planes without an unevenness, so as to contact the falling gob of the glass material only to the press surfaces; and

forming, from the gob of the glass material, a circular flat plate-shaped glass blank having a target flatness as a glass substrate for a magnetic disk such that a ratio between a diameter and a thickness of the glass blank is within range of from 50:1 to 150:1.

2. The method of manufacturing the glass blank for the substrate for the information recording medium according to claim 1 ;

wherein the pressing of the glass material by the press unit is performed through moving the press unit from a first condition, in which a distance between the press surfaces of the pair of molds is greater than a size in a horizontal direction of the gob of the glass material, to a second condition, in which the distance between the press surfaces of the pair of molds substantially coincides with a size in a direction of the thickness of the glass blank, within 0.1 second after the gob of the glass material contacts one of the pair of molds.

3. The method of manufacturing the glass blank for the substrate for the information recording medium according to claim 1 ;

wherein temperatures of the pair of molds are regulated to be less than a glass transition temperature of the glass material.

4. The method of manufacturing the glass blank for the substrate for the information recording medium according to claim 1 ;

wherein the molten glass material discharged from the glass material outlet is cut and the falling gob of the glass material is formed through moving cutting blades placed below the molten glass outlet, so that the cutting blades intersect with the glass material.

5. The method of manufacturing the glass blank for the substrate for the information recording medium according to claim 1 ;

wherein surface roughness of the press surfaces is within a range of from 0.01 to 10 μm.

6. The method of manufacturing the glass blank for the substrate for the information recording medium according to claim 1 ;

wherein a viscosity of the falling gob of the glass material is within a range of from 50 to 1050 dPa·s.

7. A method of manufacturing a substrate for an information recording medium, comprising:

manufacturing a glass blank according to the method of claim 1 ; and

grinding the glass blank to be shaped in a disk of a true circle with a predetermined diameter; and

polishing the ground glass blank.

8. A method of manufacturing an information recording medium, comprising:

manufacturing a substrate for the information recording medium according to the method of claim 7 ; and

forming information recording layers on the substrate for the information recording medium.