Method for manufacturing glass blank for magnetic disk, method for manufacturing glass substrate for magnetic disk
A method for manufacturing a glass substrate for magnetic disk is provided in which a glass is kept from being fused to a mold during press forming and shape processing to achieve a good circularity is efficiently performed. The method includes: a forming process of forming a disk-shaped glass blank by direct-pressing a molten glass by a pair of dies; and a shape processing process of performing at least one of inner hole formation and outer shape formation for forming a disk-shaped glass substrate by forming a cutting line on the principal face of the glass blank, followed by growing the cutting line to perform cutting. In the forming process, press forming is performed while the temperature of the pair of dies, over a period of time until a molten glass is separated from the die after coming into contact with the die, is set at a temperature lower than a glass transition point (Tg) and a mold release material is not attached to the surfaces of the pair of dies.
1. A method for manufacturing a glass substrate for magnetic disk, the method comprising:
a forming process of forming a disk-shaped glass blank by direct-pressing a molten glass by a pair of dies, wherein press forming is performed while the temperature of the pair of dies, over a period of time until a molten glass is separated from the die after coming into contact with the die, is set at a temperature lower than a glass transition point (Tg) and a mold release material is not attached to the surfaces of the pair of dies; and
a shape processing process including:
scribing an outside concentric circle and an inside concentric circle on the principal face of the disk-shaped glass blank to form two-concentric-circle shape cutting lines, and
growing the circular cutting lines by partially heating the disk-shaped glass blank so that a portion outside the outside concentric circle and a portion inside the inside concentric circle are removed due to a difference in thermal expansion of the disk-shaped glass blank to form a ring-shaped glass substrate, wherein a pressing force applied to a cutting line for the outside concentric circle is higher than a pressing force applied to a cutting line for the inside concentric circle.
2. The method for manufacturing a glass substrate for magnetic disk according to claim 1 , wherein the arithmetic mean roughness (Ra) of contact surfaces of the pair of dies that are in contact with the molten glass is 0.5 μm or less.
3. The method for manufacturing a glass substrate for magnetic disk according to claim 2 , wherein the arithmetic mean roughness (Ra) of the contact surfaces is 0.1 μm or less.
4. The method for manufacturing a glass substrate for magnetic disk according to claim 1 , wherein in the forming process, a falling lump of molten glass is press-formed in a direction orthogonal to a falling direction using the pair of dies.
5. The method for manufacturing a glass substrate for magnetic disk according to claim 1 , wherein, in the forming process, press forming is performed so that the temperature of portions of the pair of dies, each contacts the molten glass, is substantially identical.
6. The method for manufacturing a glass substrate for magnetic disk according to claim 1 , wherein in the shape processing process, the outside concentric circle and the inside concentric circle are scribed at the same time.