Hoya Corporation (Tokyo, JP)

A method of manufacturing a glass substrate for magnetic disc is disclosed. It includes the following steps: Dropping process for dropping a lump molten glass; Pressing process to form sheet glass by sandwiching the lump of both ends of the dropping path with the sides of the pair on opposite sides; pressing to form the lump, and then moving the pair to the direction of dropping of the bulk and a Machining process to machine sheets of glass. The press makes the lump, while moving the pair to create the mass. This assures that the relative speed of the dies is nearly zero

In light of the previous, in order to improve flatness of disk-shape glass material after being formed in the press the inventor has researched methods that include “dropping process to drop a lump of hot glass, and pressing process for making sheets of glass by sandwiching simultaneously the lump from both sides of the dropping path of the lump, using surfaces of the pair of dies that are facing each other, and performing press forming to the lump”. The using a mold release agent is not required and the flatness may be improved because the temperature difference is unlikely to be experienced between the two sets of dies.

But, as the research of the inventor the matter, it becomes evident that a made disk-shaped glass material is the shape of a disk whose length in the direction of the drop’s axial direction is more than lengths in the opposite directions. To make use of this unusual glass to serve as a substrate to make magnetic disks a process known as Scribing Process is needed. It involves cutting the substrate, which has cracks that grow from one side of the principal surface. The glass substrate is then machined to form a disk. Since the residual stress that is in-plane for irregular glass is not uniform, crack formation is difficult during the scribing process. Then, problems occur that chips are created at the outer circumference of thedisk-shaped glass substrate and the yield rate is remarkably reduced. So, when making a disk-shaped glass material using this method It is required that the glass material be substantially complete round.

Furthermore, making the glass material almost complete reduces the allowance for machining to shape and also reduces material waste. This can result in a reduction in costs.

The outer circumference of a disc-shaped glass material does not require machine work when it has a radius of 10.mu.m. In this case, elution (alkali elution for example) of the glass part from the outercircumference part may be slowed.

On the other hand it is now clear that, when cutting glass molten with cutting blades to release an unmolten lump of glass, a cut may remain in the sheet glass as well as the cut can remain as a defect of the machinedglass substrate for magnetic disk. This issue is clearly shown. It is possible to ameliorate the situation by decreasing the viscosity in glass during the process of press-forming. It is possible to lower the pressure on the press by decreasing theviscosity. This will allow for easier machining and higher efficiency.

However, it has become obvious that when the viscosity of glass diminishes, the “shape of which length in the axial direction of drop is greater than lengths in other directions” is now quite remarkable.

The invention described hereinafter offers a method for efficiently creating a glass substrate to create magnetic disks that are extremely smooth.

To solve the above issue, manufacturing of a glass substrate for magnetic disk according to the invention described herein is a process for the manufacturing of a glass surface for magnetic disk that includes a pair of principal surfaces, and the method comprises the steps of: dropping process for dropping the lump of molten glass press process for making sheets of glass by sandwiching the lump on each side of the dropping path of the lump by putting the surfaces of the pair ofdies facing each other, and then making a press-formed lump; and machining process for machining the sheet glass material wherein the press forming is able to sandwich the lump while moving the two dies towards the direction of drop of the lump so thatrelative speed of the lump to that of the dies in the pair is near zero.

The preferred speed of the lump and the pair of dies is negative in terms of the velocity component in the direction in which the lump is falling.

It is recommended that the dies’ temperatures are less than the strain point of the glass.

Preferably, the glass contains, as converted based on the oxide, denoted as molar percentages: 50 to 75% SiO.sub.2, 1 to 15% Al.sub.2O.sub.3, totally 5 to 35% at lease one component selected from the group of Li.sub.2O, Na.sub.2O, and K.sub.2O,totally 0 to 20% at lease one component selected from the group of MgO, CaO, SrO, BaO, and ZnO, and totally 0 to 10% at lease one component selected from the group of ZrO.sub.2, TiO.sub.2, La.sub.2O.sub.3, Y.sub.2O.sub.3, Ta.sub.2O.sub.5,Nb.sub.2O.sub.5, and HfO.sub.2.

Viscosity should be between 500 and 1,050 poise.

Preferably, the machining process comprises grinding process to grind the principal surfaces by using Abrasive grains fixed in place, and polishing process for polishing the primary surfaces using loose abrasive grains following the grinding process.

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