JP2012158513A - Method for manufacturing glass substrate for magnetic disk - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for efficiently manufacturing a glass substrate for a magnetic disk with high circularity.SOLUTION: The method for manufacturing the glass substrate G for the magnetic disk, the glass substrate having a pair of principal surfaces, includes: a falling process for falling a molten glass gob GG; a pressing process for sandwiching and press-molding the gob GG at the same timing from both sides of the falling route of the gob with surfaces 121a, 122a of a pair of molds 121, 122 facing each other, to mold the planar glass substrate G; and a machining process for machining the planar glass substrate G. As for a velocity component in the falling direction of the gob GG, in order to approximate the relative velocity of the gob GG to the pair of molds 121, 122 to 0, the pressing process sandwiches the gob GG while moving the pair of molds 121, 122 in the direction in which the gob GG falls.

Description

  The present invention relates to a method for manufacturing a glass substrate for a magnetic disk having a pair of main surfaces.

  Today, a personal computer, a notebook personal computer, a DVD (Digital Versatile Disc) recording device, or the like has a built-in hard disk device for data recording. In particular, in a hard disk device used in a portable computer such as a notebook personal computer, a magnetic disk in which a magnetic layer is provided on a glass substrate is used, and the magnetic head slightly floats above the surface of the magnetic disk. Magnetic recording information is recorded on or read from the magnetic layer by a (DFH (Dynamic Flying Height) head). As the substrate of this magnetic disk, a glass substrate is preferably used because it has a property that it is less likely to undergo plastic deformation than a metal substrate or the like.

Further, in response to a request for an increase in storage capacity in a hard disk device, the density of magnetic recording has been increased. For example, the magnetic recording information area is miniaturized by using a perpendicular magnetic recording method in which the magnetization direction in the magnetic layer is perpendicular to the surface of the substrate. Thereby, the storage capacity of one disk substrate can be increased. In addition, in order to further increase the storage capacity, the flying distance from the magnetic recording surface of the magnetic head is extremely shortened to reduce the magnetic recording information area. In such a magnetic disk substrate, the magnetic layer is formed flat so that the magnetization direction of the magnetic layer is substantially perpendicular to the substrate surface. For this reason, the surface irregularities of the glass substrate are made as small as possible.
Further, since the flying distance of the magnetic head is short, a head crash failure or a thermal asperity failure is likely to occur. Since these obstacles are caused by minute unevenness or particles on the magnetic disk surface, the surface unevenness of the end surface of the glass substrate is made as small as possible in addition to the main surface of the glass substrate.

  By the way, the glass substrate used for a magnetic disk is manufactured by the following method, for example. Specifically, in this method, a glass gob (a lump of glass material) made of molten glass is supplied onto a lower mold that is a receiving gob forming mold, and an upper mold that is an opposing gob forming mold is used. A glass gob is press-molded to produce a sheet glass material, and then processed into a glass substrate for an information recording medium (Patent Document 1).

  In this method, after the glass gob made of molten glass is supplied onto the lower mold, the lower surface of the upper mold cylinder and the upper surface of the lower mold mold are brought into contact with each other, and the sliding surface between the upper mold and the upper mold cylinder A thin sheet glass forming space is formed outside the sliding surface between the lower die and the lower die body, and the upper die is lowered and press-molded, and the upper die is raised immediately after press molding. . Thereby, the plate-shaped glass raw material used as the origin of the glass substrate for magnetic discs is shape | molded. Thereafter, a glass substrate for a magnetic disk is obtained through a grinding process and a polishing process.

In the grinding step, for example, grinding using alumina-based loose abrasive grains is performed. In this step, the first grinding step and the second grinding step are performed using loose abrasive grains having different particle sizes. The particle size of the loose abrasive used in the second grinding step is set smaller than the particle size of the loose abrasive used in the first grinding step. Thereby, rough grinding and fine grinding are performed in this order.
The polishing step includes, for example, a first polishing step using free abrasive grains such as cerium oxide and a hard resin material polisher, and a second polishing step using, for example, colloidal silica and a soft resin material polisher. The grain size of the abrasive grains used in the first polishing process is smaller than the grain size of the abrasive grains used in the second grinding process during the grinding process. Furthermore, the particle size of the abrasive grains used in the second polishing step is smaller than the particle size of the abrasive grains used in the first polishing step.

  As described above, in the surface processing on the glass substrate, the first grinding step, the second grinding step, the first polishing step, and the second polishing step are performed in this order, and the surface quality such as the surface roughness of the glass substrate is improved. Process to gradually improve.

Japanese Patent No. 3770933

However, the surface accuracy of the glass substrate formed by the above-described method is not sufficient for the surface accuracy of the main surface required for increasing the density of the magnetic recording and miniaturizing the magnetic recording information area.
For example, when forming a sheet glass material, a mold release agent is applied to the mold surface in order to prevent the glass material from fusing to the mold surfaces of the upper mold and the lower mold, and this mold release agent is used. In particular, the surface roughness of the main surface of the sheet glass material is large. Further, the difference in surface temperature between the upper mold and the lower mold is large, and the lower mold to which the glass bump (a lump of glass material) is supplied becomes high temperature. Because this surface temperature difference creates a temperature distribution in the thickness direction of the molded sheet glass material and in the plane of this sheet, the amount of shrinkage of the sheet glass material taken out of the mold and cooled is also the sheet glass material. Distribution in the thickness direction and in the plane of this plate. For this reason, the glass sheet material is easily warped, and as a result, the flatness of the glass sheet material when formed is low.

The flatness of such a sheet glass material can be improved by grinding (first grinding step). For example, in order to improve the flatness, the machining allowance (the amount of cutting) in the grinding process is increased. However, if the machining allowance in the grinding process is increased, deep cracks enter the surface of the sheet glass material. Therefore, in order to prevent deep cracks from remaining, the machining allowance (polishing amount) is inevitably also in the subsequent polishing process. growing. However, if the machining allowance is increased in the polishing process using loose abrasive grains and a resin polisher, the vicinity of the outer peripheral edge portion of the main surface of the plate-like glass material is rounded off, and the “sag problem” occurs at the edge portion. That is, since the vicinity of the outer peripheral edge portion of the sheet glass material is rounded, when the magnetic disk is produced using this sheet glass material as a glass substrate, the flying distance of the magnetic head near the outer edge portion on the main surface is reduced. , Larger than the flying distance of the magnetic head in another part on the main surface. Further, since the vicinity of the outer peripheral edge portion has a rounded shape, surface irregularities are generated. As a result, the flying distance of the magnetic head becomes unstable in the vicinity of the outer peripheral edge, and the recording and reading operations may not be performed accurately, or the magnetic head and the magnetic disk may collide so that reading and writing are impossible. This is "who's problem".
Further, since the machining allowance in the polishing process becomes large, the polishing process is not practically preferable because it takes a long time.

  Therefore, in order to improve the flatness of the plate (disk) glass material after the press molding, the present inventor "opposite each other from both the dropping step of dropping the molten glass lump and the dropping path of the lump. And a pressing step of forming a plate-like glass having a pair of main surfaces facing each other by sandwiching the lump between the surfaces of a pair of molds at the same timing and performing press molding. In this method, it is not necessary to use a mold release agent, and a flatness can be improved because a temperature difference hardly occurs between a pair of molds.

  However, as the inventor has repeatedly studied, it has become clear that the shape of the disc-shaped glass material to be produced has a longer drop axis direction than the other axis direction. When such an irregular glass is used as a glass substrate for a magnetic disk, for example, it is cut by growing a crack from one main surface side to the other main surface side in a process called a scribing process. It is necessary to process it into a disk shape. However, because the in-plane residual stress is particularly uneven in the irregular glass, the direction of crack growth is unstable due to the scribing process, and the outer periphery of the disk-shaped glass substrate is chipped. The problem arises that the yield is significantly reduced. Therefore, in producing a disk-shaped glass material by the above method, it is necessary to make the shape close to a perfect circle.

In addition, since the amount of processing of the outer shape is reduced by bringing the shape of the disk-shaped glass material closer to a perfect circle, less material is wasted, leading to cost reduction.
Furthermore, when the roundness of the disk-shaped glass material is, for example, 10 μm or less, it is not necessary to process the outer peripheral portion, so that elution of glass components from the outer peripheral portion (for example, alkali elution) can be suppressed.

On the other hand, when cutting the molten glass using a cutting blade to drop the molten glass lump, cutting marks may remain in the plate glass material, and this is the glass for magnetic disk after processing. It has also been found that it remains as a substrate defect. When this point was examined, it has been found that the glass can be improved by lowering the viscosity of the glass during press molding. Note that lowering the viscosity of the glass leads to lowering the press pressure, and thus has the advantage of being easy to work and improving productivity.
However, as the viscosity of the glass is lowered, it has become clear that the above-mentioned “the shape of the disk-shaped glass material has a longer drop axis direction than the other axis direction” becomes more prominent.

  Therefore, an object of the present invention is to provide a method for efficiently producing a glass substrate for a magnetic disk having excellent roundness.

  In order to solve the above problems, a method for manufacturing a glass substrate for a magnetic disk of the present invention is a method for manufacturing a glass substrate for a magnetic disk having a pair of main surfaces, and a dropping step of dropping a molten glass lump, From both sides of the lump dropping path, the lump is sandwiched and pressed at the same timing between the surfaces of a pair of molds facing each other, and the sheet glass material is processed by pressing. And a pressing step is performed in the direction in which the lump falls in order to bring the relative speed of the lump with respect to the pair of dies close to 0 with respect to the velocity component in the lump falling direction. The mass is sandwiched while moving.

  Moreover, it is preferable that the relative velocity of the lump with respect to the pair of molds is set to 0 for the velocity component in the dropping direction of the lump.

  Moreover, it is preferable that the temperature of the pair of molds is a temperature below the strain point of the glass.

Moreover, when said glass is converted into oxide standards, it is expressed in mol%, and SiO ₂ is 50 to 75%, Al ₂ O ₃ is 1 to 15%, Li ₂ O, Na ₂ O and K ₂ O. 5 to 35% in total of at least one selected component, 0 to 20% in total of at least one component selected from MgO, CaO, SrO, BaO and ZnO, and ZrO ₂ , TiO ₂ , It is preferably composed of a composition having 0 to 10% in total of at least one component selected from La ₂ O ₃ , Y ₂ O ₃ , Ta ₂ O ₅ , Nb ₂ O ₅ and HfO ₂ .

  Moreover, it is preferable that the viscosity of the molten glass is in the range of 500 poise to 1050 poise.

  The processing step preferably includes a grinding step of grinding the main surface using fixed abrasive grains, and a polishing step of polishing the main surface using loose abrasive grains after the grinding step. .

  According to the method for producing a glass substrate for magnetic disk of the present invention, a glass substrate for magnetic disk having excellent roundness can be produced efficiently.

(A) is a schematic block diagram which shows an example of the magnetic disc produced using the glass substrate for magnetic discs, (b) is sectional drawing of a magnetic disc, (c) is a magnetic head magnetically It is a figure which shows the state which floats on the surface of a disk. It is a figure which shows the flow of one Embodiment of the manufacturing method of the glass substrate for magnetic discs of this invention. It is a top view of the apparatus used in press molding. (A) is a side view before a molten glass and a cutting unit contact, (b) is a side view after a cutting unit cut out a molten glass, (c) is a press unit being a molten glass It is a side view of the state which is press-molding the lump. (A) is a whole figure of the apparatus used for grinding by fixed abrasive, (b) is a figure explaining the carrier used for the apparatus shown to (a). It is a figure explaining the state when grinding a plate-shaped glass raw material. (A) is a side view of the modified example 1 before a molten glass and a cutting unit contact, (b) is a side view after a cutting unit cut out molten glass, (c) is a press It is a side view of the state in which the unit is press-molding the lump of molten glass. (A), (b) is a side view which shows the state before the lump of molten glass is made, (c) is a side view which shows the state where the lump of molten glass was made by the gob formation type | mold. (D) is a side view showing a state where the press unit presses a lump of molten glass. (A) is a side view before dropping the lump of optical glass, (b) is a side view of the state where the lump of optical glass is falling, and (c) is a side view when the press unit is optical glass. It is a side view of the state which is press-molding the lump.

Hereinafter, the manufacturing method of the glass substrate for magnetic disks of this invention is demonstrated in detail.
(Magnetic disk and glass substrate for magnetic disk)
First, a magnetic disk manufactured using a magnetic disk glass substrate will be described with reference to FIG. FIG. 1A is a schematic configuration diagram showing an example of a magnetic disk manufactured using a magnetic disk glass substrate. FIG. 1B is a schematic cross-sectional view of a magnetic disk. FIG. 1C is a diagram showing a state in which the magnetic head floats on the surface of the magnetic disk.

As shown in FIG. 1A, the magnetic disk 1 has a ring shape and rotates about a rotation axis. As shown in FIG. 1B, the magnetic disk 1 includes a glass substrate 2 and at least magnetic layers 3A and 3B. The magnetic layers 3 </ b> A and 3 </ b> B are formed on the main surface of the ring-shaped glass substrate 2.
In addition to the magnetic layers 3A and 3B, for example, an adhesion layer, a soft magnetic layer, a nonmagnetic underlayer, a perpendicular magnetic recording layer, a protective layer, a lubricating layer, and the like (not shown) are formed. For the adhesion layer, for example, a Cr alloy or the like is used. The adhesion layer functions as an adhesive layer with the glass substrate 2. For the soft magnetic layer, for example, a CoTaZr alloy or the like is used. For example, a Ru alloy or the like is used for the nonmagnetic underlayer. For example, a granular magnetic layer is used for the perpendicular magnetic recording layer. For the protective layer, for example, a material made of hydrogenated carbon is used. For the lubricating layer, for example, a fluorine-based resin or the like is used.

The magnetic disk 1 will be described using a more specific example. In the present embodiment, a CrTi adhesion layer, a CoTaZr / Ru / CoTaZr soft magnetic layer, a Ru underlayer, and CoCrPt—SiO ₂ · TiO ₂ granular magnetism are formed on both main surfaces of the glass substrate 2 using a sputtering apparatus. A layer and a hydrogenated carbon protective film are sequentially formed. Further, a perfluoropolyether lubricating layer is formed on the formed uppermost layer by dipping.

When the magnetic disk 1 is used in a hard disk device, the magnetic disk 1 rotates around a rotation axis at a rotation speed of 7200 rpm, for example. As shown in FIG. 1C, each of the magnetic heads 4 </ b> A and 4 </ b> B of the hard disk device floats by a distance H from the surface of the magnetic disk 1 as the magnetic disk 1 rotates at high speed. The distance H at which the magnetic heads 4A and 4B fly is, for example, 5 nm. In this state, the magnetic heads 4A and 4B record or read information on the magnetic layer. By flying the magnetic heads 4A and 4B, the magnetic recording information area can be miniaturized and magnetic recording can be performed by performing recording or reading on the magnetic layer at a short distance without sliding with respect to the magnetic disk 1. Achieve high density.
At this time, the magnetic heads 4A and 4B are accurately operated from the central portion of the glass substrate 2 of the magnetic disk 1 to the outer peripheral edge portion 5 with the target surface accuracy and with the distance H = 5 nm maintained. be able to.
As described later, the processing of the surface unevenness of the glass substrate 2 includes grinding using fixed abrasive grains having a small machining allowance, and, as a result, first polishing and second polishing capable of reducing the machining allowance. It is made after.

  The surface roughness of the main surface of the glass substrate 2 used for such a magnetic disk 1 has a flatness of, for example, 4 μm or less and a surface roughness of, for example, 0.2 nm or less. The flatness of 4 μm or less is a target flatness required for a magnetic disk substrate as a final product.

The flatness can be measured using, for example, a flatness tester FT-900 manufactured by Nidec.
The roughness (Ra) of the main surface is obtained, for example, by measuring a range of 1 μm × 1 μm with a resolution of 512 × 256 pixels using a scanning probe microscope (atomic force microscope) manufactured by SII Nano Technologies. It can be the arithmetic average Ra of the surface roughness obtained.

As a material of the glass substrate 2, aluminosilicate glass, soda lime glass, borosilicate glass, or the like can be used. In particular, aluminosilicate glass can be suitably used in that it can be chemically strengthened and a glass substrate for a magnetic disk excellent in the flatness of the main surface and the strength of the substrate can be produced.
As aluminosilicate glass, SiO ₂ is 50 to 75%, Al ₂ O ₃ is 1 to 15%, and at least one component selected from Li ₂ O, Na ₂ O and K ₂ O is added in mol%. 5 to 35%, at least one component selected from MgO, CaO, SrO, BaO and ZnO in total 0 to 20%, and ZrO ₂ , TiO ₂ , La ₂ O ₃ , Y ₂ O ₃ , It is preferable to use a total of 0 to 10% of at least one component selected from Ta ₂ O ₅ , Nb ₂ O ₅ and HfO ₂ . Further, as aminosilicate glass, SiO ₂ is 57 to 74%, ZnO ₂ is 0 to 2.8%, Al ₂ O ₃ is ₃ to 15%, Li ₂ O is 7 to 16%, Na in terms of mol%. _A glass material for chemical strengthening containing 4 to 14% of ₂ O as a main component can also be used.

(Method for producing glass substrate for magnetic disk)
Next, with reference to FIG. 2, the flow of the manufacturing method of the glass substrate for magnetic discs is demonstrated. FIG. 2 is a diagram showing a flow of an embodiment of a method for manufacturing a glass substrate for magnetic disk.
As shown in FIG. 2, first, a sheet glass material is produced by press molding (step S10). Next, the formed sheet glass material is scribed (step S20). Next, the scribed plate-shaped glass material is processed (step S30). Next, the sheet glass material is ground with fixed abrasive grains (step S40). Next, end face polishing of the sheet glass material is performed (step S50). Next, 1st grinding | polishing is given to the main surface of a sheet glass raw material (step S60). Next, chemical strengthening is performed on the plate-shaped glass material after the first polishing (step S70). Next, second polishing is performed on the chemically strengthened sheet glass material (step S80).
Hereinafter, each step will be described in detail.

(A) Press molding process First, with reference to FIG. 3, a press molding process (step S10) is demonstrated. FIG. 3 is a plan view of an apparatus used in press molding. As shown in FIG. 3, the apparatus 101 includes four sets of press units 120, 130, 140, 150 and a cutting unit 160.
The cutting unit 160 is provided on a path through which the molten glass flows out from the molten glass outlet 111. When the molten glass is cut by the cutting unit 160, the lump of molten glass falls downward in the vertical direction. The press units 120, 130, 140, and 150 form a sheet glass material by sandwiching and pressing the lump at the same timing from a pair of mold surfaces facing each other from both sides of the lump dropping path.
In the example shown in FIG. 3, the four sets of press units 120, 130, 140, and 150 are provided every 90 degrees around the molten glass outlet 111.
Here, the melted glass preferably has a viscosity of 50 poise to 1050 poise. With such a configuration, the glass material can be formed into a desired shape in a short time. If the melted glass has a viscosity of less than 50 poise, it is difficult to form a lump of glass material, and if it exceeds 1050 poise, it becomes difficult to form a thin glass blank with a short press. Further, the viscosity of the molten glass is more preferably in the range of 500 poise to 1050 poise.

  Each of the press units 120, 130, 140, and 150 is driven by a moving mechanism (not shown) and can advance and retreat with respect to the molten glass outlet 111. That is, a catch position (a position where the press unit 140 is drawn with a solid line in FIG. 3) located immediately below the molten glass outlet 111 and a retreat position (the press unit 120 in FIG. 3) away from the molten glass outlet 111. , 130 and 150 are movable between a position drawn with a solid line and a position where the press unit 140 is drawn with a broken line.

The cutting unit 160 is provided on the molten glass path between the catch position and the molten glass outlet 111. The cutting unit 160 cuts out an appropriate amount of molten glass flowing out from the molten glass outlet 111 to form a molten glass lump (hereinafter also referred to as a gob). The cutting unit 160 has a first cutting blade 161 and a second cutting blade 162.
The first cutting blade 161 and the second cutting blade 162 are driven to intersect on the molten glass path at a fixed timing. When the 1st cutting blade 161 and the 2nd cutting blade 162 cross | intersect, molten glass is cut out and a gob is obtained. The obtained gob falls vertically downward toward the catch position.

The press unit 120 includes a first die 121, a second die 122, a first drive unit 123, and a second drive unit 124.
Each of the first mold 121 and the second mold 122 is a plate-like member having a surface for press-molding the gob. The normal direction of the two surfaces is a substantially horizontal direction, and the two surfaces are arranged to face each other in parallel.

The first drive unit 123 moves the first mold 121 forward and backward with respect to the second mold 122. On the other hand, the second drive unit 124 moves the second mold 122 forward and backward with respect to the first mold 121. The first drive unit 123 and the second drive unit 124 have a mechanism for rapidly bringing the surface of the first mold 121 and the surface of the second mold 122 into close proximity. The 1st drive part 123 and the 2nd drive part 124 are the mechanisms which combined the air cylinder, the solenoid, and the coil spring, for example.
Note that the structure of the press units 130, 140, and 150 is the same as that of the press unit 120, and a description thereof will be omitted.

  After each press unit moves to the catch position, the falling gob is sandwiched between the first die and the second die by the drive of the first drive unit and the second drive unit, and formed into a predetermined thickness. At the same time, it is rapidly cooled to produce a circular plate-like glass material G. Thereafter, the press unit moves to the retracted position, pulls the first mold and the second mold apart, and drops the formed sheet glass material G.

  A first conveyor 171, a second conveyor 172, a third conveyor 173, and a fourth conveyor 174 are provided below the retreat positions of the press units 120, 130, 140, and 150, respectively. Each of the first to fourth conveyors 171 to 174 receives the sheet glass material G falling from the corresponding press unit, and conveys the sheet glass material G to the next process apparatus (not shown).

In the present embodiment, the press units 120, 130, 140, and 150 are configured to sequentially move to the catch position and sandwich the gob and move to the retreat position. Therefore, the glass sheet G can be continuously formed without waiting for the cooling of the glass sheet G in each press unit.
Note that it is also possible to form the sheet glass material G by using only one press unit 120 and sandwiching the gob continuously. In this case, the first mold 121 and the second mold 122 are opened immediately after the gob _GG is press-molded, and the molten glass lump that falls next is press-molded.

Here, with reference to the side view shown by FIG. 4, the press molding process of this embodiment is demonstrated. 4 (a) is a side view before the molten glass _{L G} and the cutting unit 160 is in contact. FIG. 4 (b), the cutting unit 160 is a side view after cutting out the molten glass _{L G.} Figure 4 (c) is a side view illustrating a state where the pressing unit 120 is press-molded mass G _G of the molten glass.

As shown in FIG. 4 (a), molten glass material L _G is continuously flowing out from the molten glass outflow port 111. As shown in FIG. 4 (b), to drive the cutting unit 160 at predetermined timing, the first cutting blade 161 by the second cutting blade 162 to cut the molten glass material _{L G.} Thereby, the cut | disconnected molten glass becomes a substantially spherical gob _GG with the surface tension. 4 In the example shown in (b), each time for driving the cutting unit 160, for example, as a gob G _G having a diameter of about 15mm is formed, runoff and cutting unit per unit time of the molten glass material L _G The driving interval of 160 is adjusted.

At the timing when the gob _{G G} enters the first die 121 in the gap of the second die 122, in the direction of the gob _{G G} drops with the first die 121 and second die 122 is moved, the first die 121 and the second die 122 so as to sandwich the gob _{G G,} a first driving unit 123 and the second driving unit 124 is driven. Specifically, the first mold 121 moves in the direction of the arrow indicated by A in FIG. The second mold 122 moves in the direction of the arrow indicated by B in FIG. Thereby, as shown in FIG. 4C, the gob _GG is captured (caught) between the first mold 121 and the second mold 122.

  Note that the inner peripheral surface of the second mold 122 is formed so as to form a plate-like space while maintaining a constant distance between the inner peripheral surface 121a of the first mold 121 and the inner peripheral surface 122a of the second mold 122. A protruding spacer 122b is provided on 122a.

The first mold 121 and the second mold 122 are provided with a temperature adjustment mechanism (not shown). The temperature of the first die 121 and second die 122 is a temperature adjusting mechanism, it is preferably adjusted to a temperature lower than a strain point of the molten glass L _G.

From the gob _{G G} is in contact with the inner peripheral surface 122a of the inner circumferential surface 121a or the second die 122 of the first die 121, the first die 121 and second die 122 is gob _{G G} complete The time required for confinement is as short as about 0.06 seconds. Therefore, the gob G _G is formed within a very short period of time along the inner circumferential surface 121a and the inner circumferential surface 122a of the second die 122 of the first die 121 to spread substantially circular, further rapidly cooled And solidifies as amorphous glass. Thereby, the sheet glass material G is produced.
In addition, the plate-shaped glass raw material G shape | molded in this embodiment is a circular board of diameter 75-80mm and thickness about 1mm, for example.

After the first mold 121 and the second mold 122 are closed, the press unit 120 quickly moves to the retracted position. Subsequently, other press unit 130 is moved to the catch position by the pressing unit 130, press the gob G _G is performed.

  After the press unit 120 moves to the retracted position, the first mold 121 and the second mold 122 are closed until the sheet glass material G is sufficiently cooled (for example, until the temperature becomes lower than the bending point). To maintain the state. Thereafter, the first driving unit 123 and the second driving unit 124 are driven to separate the first mold 121 and the second mold 122, and the sheet glass material G drops off the press unit 120 and falls below Is received by the conveyor 171 (see FIG. 3).

  In the present embodiment, the first mold 121 and the second mold 122 are closed within an extremely short time of 0.1 seconds (about 0.06 seconds), and the inner peripheral surface 121a of the first mold 121 is closed. The molten glass contacts the entire inner peripheral surface 122a of the second mold 122 almost simultaneously. For this reason, local heating of the inner peripheral surface 121a of the first mold 121 and the inner peripheral surface 122a of the second mold 122 is suppressed, and almost no distortion occurs in the inner peripheral surface 121a and the inner peripheral surface 122a. Absent.

Moreover, before the heat is transferred to the first die 121 and second die 122 from the gob G _G, since the gob G _G is formed into a circular shape, the temperature distribution of the sheet glass material G to be shaped is substantially It becomes uniform. For this reason, when the molten glass is cooled, the distribution of the shrinkage amount of the glass material is small, and the distortion of the sheet glass material G hardly occurs. Therefore, the flatness of the main surface of the produced plate glass material G is improved as compared with the plate glass material produced by the conventional press molding, and the target flatness of the main surface necessary as a glass substrate for a magnetic disk. Can be.

  The surface roughness of the inner peripheral surface 121a and the inner peripheral surface 122a is preferably 0.01 μm to 1 μm so that the arithmetic average roughness Ra of the sheet glass material G is 0.01 μm to 10 μm. So that it can be adjusted.

Here, generally, the press unit in the direction of the gob G _G drops without moving the press unit may sandwich the gob G _G, the relative speed of the gob G _G increases for press unit. By gob G _G is sandwiched pressing unit while moving in the vertical direction, the plate-like glass material G thus manufactured, the length of the vertical direction is long elliptical shape as compared to the horizontal direction.
In contrast, in the present embodiment, at the timing when the gob G _G enters the press unit, while moving the pressing unit in the direction of the gob G _G drops pressing unit for sandwiching the gob G _G, of the gob G _G against the press unit The relative speed is reduced. Thereby, the roundness of the produced sheet glass material G can be improved. By improving the roundness of the sheet glass material G, the machining allowance in the chamfering process to be described later can be reduced, and the generation of cracks in the grinding process and the polishing process can be suppressed.

Further, at the timing of contact between the press unit and the gob G _G, so as to have the same rate as the gob G _G drops, i.e., as the relative speed of the gob G _G against the press unit is 0, gob G It is preferable to move the press unit in the direction in which _G drops.

(B) Scribe process Next, a scribe process (step S20) is demonstrated. After the press molding process, in the scribing process, scribing is performed on the formed sheet glass material G.
Here, the scribing means that the formed glass sheet G has a ring shape of a predetermined size, and two concentric circles (inner side) are formed on the surface of the glass sheet G by a scriber made of super steel alloy or diamond particles. Concentric circles and outer concentric circles) are provided with cutting lines (linear flaws). The sheet glass material G scribed in the shape of two concentric circles is partially heated, and due to the difference in thermal expansion of the sheet glass material G, the outer portion of the outer concentric circle and the inner portion of the inner concentric circle are removed. Thereby, it becomes a ring-shaped plate-shaped glass material.
In addition, it is also possible to make the plate glass material into a ring shape by setting the outer diameter and roundness to such an extent that no scribing is necessary, and forming a circular hole using such as a core drill on such a plate glass material it can.

(C) Shape processing step (chambering step)
Next, the shape processing step (step S30) will be described. In the shape processing step, shape processing of the scribed plate-shaped glass material G is performed. Shape processing includes chamfering (chamfering of the outer peripheral end and the inner peripheral end).
The outer peripheral end and the inner peripheral end of the ring-shaped sheet glass material G are chamfered with a diamond grindstone.

(D) Grinding process using fixed abrasive grains Next, the grinding process using fixed abrasive grains (step S40) will be described. In the grinding process using the fixed abrasive grains, the ring-shaped sheet glass material G is ground using the fixed abrasive grains. The machining allowance by grinding with fixed abrasive grains is, for example, about several μm to 100 μm. The particle size of the fixed abrasive is, for example, about 10 μm.

Here, with reference to FIG.5 and FIG.6, the process of grinding the sheet glass raw material G is demonstrated. FIG. 5A is an overall view of an apparatus used for grinding with fixed abrasive grains. FIG. 5B is a diagram showing a carrier used in this apparatus. FIG. 6 is a diagram for explaining a state in which the plate-like glass material G is being ground.
As shown in FIGS. 5A and 6, the apparatus 400 includes a lower surface plate 402, an upper surface plate 404, an internal gear 406, a carrier 408, a diamond sheet 410, a sun gear 412, an internal gear. A null gear 414, a container 416, and a pump 420 are included. Further, the container 416 has a coolant 418.

The apparatus 400 sandwiches an internal gear 406 between the lower surface plate 402 and the upper surface plate 404 from above and below. A plurality of carriers 408 are held in the internal gear 406 during grinding. In the example shown in FIG. 5B, the internal gear 406 holds five carriers 408.
The surface of the diamond sheet 410 that is planarly bonded to the lower surface plate 402 and the upper surface plate 404 is a ground surface. That is, the plate-like glass material G is ground with fixed abrasive grains using the diamond sheet 410.

The plurality of sheet glass materials G to be ground are arranged and held in circular holes provided in each carrier 408 as shown in FIG. A pair of main surfaces of the plate-shaped glass material G are in contact with the diamond sheet 410 between the lower surface plate 402 and the upper surface plate 404 during grinding.
On the other hand, the sheet glass material G is held on the lower surface plate 402 by a carrier 408 having a gear 409 on the outer periphery. The carrier 408 meshes with a sun gear 412 and an internal gear 414 provided on the lower surface plate 402. By rotating the sun gear 412 in the direction of the arrow, each carrier 408 revolves while rotating as a planetary gear in the direction of the arrow. Thereby, the glass sheet G is ground using the diamond sheet 410.

  As shown in FIG. 5A, the apparatus 400 supplies the coolant 418 in the container 416 into the upper platen 404 by the pump 420, collects the coolant 418 from the lower platen 402, and returns it to the container 416. Circulate. At this time, the coolant 418 removes the face generated during grinding from the grinding surface. Specifically, when circulating the coolant 418, the apparatus 400 performs filtration with the filter 422 provided in the lower surface plate 402, and retains the facets in the filter 422.

  In the grinding device 400, grinding is performed using the diamond sheet 410, but instead of the diamond sheet 410, fixed abrasive grains provided with diamond particles can be used. For example, pellets obtained by bonding a plurality of diamond particles with a resin can be used for grinding with fixed abrasive grains.

(E) End surface polishing step Next, the end surface polishing step (step S50) will be described. After the grinding with the fixed abrasive, the end surface polishing of the sheet glass material G is performed in the end surface polishing step.
In the end surface polishing, mirror finishing is performed on the inner peripheral side end surface and the outer peripheral side single surface of the sheet glass material G by brush polishing. At this time, a slurry containing fine particles such as cerium oxide as free abrasive grains is used. By performing end face polishing, removal of contamination such as dust attached to the end face of the sheet glass material G, damage, or damage such as scratches, ion precipitation that causes corrosion such as sodium and potassium Can be prevented.

(F) First Polishing (Main Surface Polishing) Step Next, the first polishing step (Step S60) will be described. In the first polishing step after the end surface polishing step, the main surface of the sheet glass material G is subjected to the first polishing. The machining allowance by the first polishing is, for example, about several μm to 50 μm.
The purpose of the first polishing is to remove scratches and distortions remaining on the main surface by grinding with fixed abrasive grains. In the first polishing, the apparatus 400 used in grinding with fixed abrasive grains (step S40) is used. Unlike grinding with fixed abrasive grains, in the first polishing step, free abrasive grains turbid in the slurry are used instead of fixed abrasive grains. In the first polishing step, no coolant is used. In the first polishing process, a resin polisher is used instead of the diamond sheet 410.
As the free abrasive grains used for the first polishing, for example, fine particles (particle size: about 1 to 2 μm in diameter) such as cerium oxide suspended in the slurry are used.

(G) Chemical strengthening process Next, a chemical strengthening process (step S70) is demonstrated. After the first polishing step, in the chemical strengthening step, the plate glass material G after the first polishing is chemically strengthened.
As the chemical strengthening liquid, for example, a mixed liquid of potassium nitrate (60%) and sodium sulfate (40%) is used. In the chemical strengthening, the chemical strengthening liquid is heated to, for example, 300 ° C. to 400 ° C., and the washed sheet glass material G is preheated to, for example, 200 ° C. to 300 ° C. Then, the sheet glass material G is in the chemical strengthening liquid. For example, 3 hours to 4 hours. In the case of this immersion, it may be performed in the state accommodated in the holder so that a plurality of plate glass materials G may be held at the end face so that both main surfaces of the plate glass material G are chemically strengthened. preferable.

Thus, by immersing the sheet glass material G in the chemical strengthening solution, the lithium ions and sodium ions on the surface layer of the sheet glass material G are sodium ions and potassium having a relatively large ion radius in the chemical strengthening solution. Each is replaced by ions, and the glass sheet material G is strengthened.
Note that the chemically strengthened sheet glass material G is washed. For example, after washing with sulfuric acid, washing with pure water, IPA (isopropyl alcohol), or the like.

(H) Second Polishing (Final Polishing) Step Next, the second polishing step (Step S80) will be described. In the second polishing step, second polishing is performed on the glass sheet G that has been chemically strengthened and sufficiently cleaned. The machining allowance by the second polishing is, for example, about 1 μm.
The second polishing is intended for mirror polishing of the main surface. In the second polishing, the apparatus 400 used in the grinding with the fixed abrasive (step S40) and the first polishing (step S60) is used. In the second polishing, the type and particle size of the loose abrasive grains are different from those in the first polishing. In the second polishing, the hardness of the resin polisher is different from that in the first polishing.

As the free abrasive grains used for the second polishing, for example, fine particles (particle size: diameter of about 0.1 μm) such as colloidal silica made turbid in the slurry are used.
Thus, the polished plate glass material G is washed. In cleaning, a neutral detergent, pure water, and IPA are used.
By the second polishing, the magnetic disk glass substrate 2 having a surface irregularity with a main surface flatness of 4 μm or less and a main surface roughness of 0.2 nm or less is obtained.
Thereafter, as shown in FIG. 1, the magnetic layer layers 3 </ b> A and 3 </ b> B are formed on the magnetic disk glass substrate 2 to manufacture the magnetic disk 1.

The above is the description of the flow shown in FIG. In the flow shown in FIG. 2, the scribing (step S20) and the shape processing (step S30) are performed between the grinding with the fixed abrasive (step S40) and the first polishing (step S60). Further, chemical strengthening (step S70) is performed between the first polishing (step S60) and the second polishing (step S80).
However, the order of these steps is not particularly limited. As long as the first polishing (step S60) and then the second polishing (step S80) are performed after grinding with fixed abrasive (step S40), scribing (step S20), shape processing (step S30), and chemical strengthening (step) Each process of S70) can be arrange | positioned suitably.

  In the present embodiment, the formed plate-like glass material G is not subjected to the twice grinding (first grinding and second grinding) using the free abrasive grains as in the prior art, but once using the fixed abrasive grains. After grinding, first polishing and second polishing are performed. The reason why one grinding step can be omitted in this way is that a plate-like glass material having a target flatness required for the main surface as a magnetic disk glass substrate can be formed.

The allowance in conventional grinding (first grinding process and second grinding process) and polishing (first polishing process and second polishing process) is to eliminate the low flatness of the glass sheet material to be formed. It exceeded 150 μm. That is, the machining allowance is largely determined in the conventional first grinding step and second grinding step. When sharply cut by grinding, the flatness is improved, but the crack progresses deeply.
For this reason, the machining allowance in the first polishing and the second polishing is inevitably large. Due to such a large machining allowance in polishing, as described above, a “sag problem” occurs at the edge portion where the vicinity of the outer peripheral edge portion of the glass substrate is rounded off. Note that the reason why the vicinity of the outer peripheral edge portion is rounded is that a hard or soft resin polisher is used when performing the above-described first polishing and second polishing.

  As described above, in this embodiment, the main surface of the formed sheet glass material is ground with fixed abrasive grains, and further, the first surface is applied to the main surface of the ground sheet glass material that has been ground. Can do. For this reason, in the method of this embodiment, the machining allowance by grinding can be made small, and as a result, the machining allowance in grinding, the 1st polish, and the 2nd polish can be made into 10 micrometers-150 micrometers.

Further, in the present embodiment, by while moving the press unit in the direction of the gob G _G drops pressing unit sandwiching the gob G _G, it is possible to improve the roundness of the sheet glass material G, the result The occurrence of cracks in the grinding process and polishing process can be suppressed.

(Modification 1)
Next, the manufacturing method of the glass substrate for magnetic disks of the modification 1 is demonstrated. The manufacturing method of the glass substrate for magnetic disks of this modification differs from the embodiment described above in the press molding process. Since other processes are the same as those in the above-described embodiment, description thereof is omitted. Hereinafter, with reference to FIG. 7, the press molding process of this modification is demonstrated. 7 (a) is in the present modification is a side view before the molten glass L _G and the cutting unit 160 is in contact. FIG. 7 (b), in this modification, the cutting unit 160 is a side view after cutting out the molten glass L _G. FIG. 7 (c), in the present modification is a side view of a state where the pressing unit 120 is press-molded mass G _G of the molten glass.

The apparatus 101 according to this modification further includes a gob forming mold 180. The gob forming mold 180 includes two blocks 181 and 182. A depression is formed on the upper surface of the blocks 181 and 182, and a recess 180 c is formed on the upper surface of the gob forming mold 180 by closing the two blocks 181 and 182.
As shown in FIG. 7 (a), molten glass material L _G is continuously flowing out from the molten glass outflow port 111. By closing the block 181 and 182 along the path of the molten glass _{L G,} mass is cut by the cutting unit 160 molten glass _{L G} is received by the recess 180c.

As shown in FIG. 7 (b), to drive the cutting unit 160 at predetermined timing, the first cutting blade 161 by the second cutting blade 162 to cut the molten glass material L _G. Thus, cut molten glass is received in a gob forming mold 180, the gob G _G Summary spherical by the surface tension. Thereafter, the blocks 181 and 182 are opened, the gob _{G G} formed in the recess 180c falls vertically downward.

In this modification, at the timing when the gob G _G enters the first die 121 in the gap of the second die 122, the first die 121 in a direction gob G _G drops the second die 122 moves thereby, the first die 121 and second die 122 is so as to sandwich the gob _{G G,} a first driving unit 123 and the second driving unit 124 is driven. Specifically, the first mold 121 moves in the direction of the arrow indicated by A in FIG. The second mold 122 moves in the direction of the arrow indicated by B in FIG. Accordingly, as shown in FIG. 7C, the gob _GG is captured (caught) between the first mold 121 and the second mold 122.

According to this modification, a low viscosity of the molten glass L _G, not molten glass in was cleaved by cutting the molten glass L _G by the cutting unit 160 is substantially spherical, is to form a gob of substantially spherical Even in difficult cases, the gob can be formed by the gob forming mold 180.
Further, while moving the pressing unit in the direction of the gob G _G drops pressing unit by sandwiching the gob G _G, the relative speed of the gob G _G against the press unit is decreased. As a result, the roundness of the sheet glass material G is improved, the machining allowance in the chamfering process can be reduced, and the occurrence of cracks in the grinding process and the polishing process can be suppressed.

(Modification 2)
Next, the manufacturing method of the glass substrate for magnetic disks of the modification 2 is demonstrated. The manufacturing method of the glass substrate for magnetic disks of this modification differs from the embodiment described above in the press molding process. Since other processes are the same as those in the above-described embodiment, description thereof is omitted. Hereinafter, with reference to FIG. 8, the press molding process of this modification is demonstrated. FIGS. 8A and 8B are side views showing a state before a molten glass lump _GG is made in the present modification. FIG. 8C is a side view showing a state in which a molten glass lump _GG is made by the gob forming mold 180 in the present modification. FIG. 8 (d) in the present modification is a side view showing a state where the pressing unit 120 is a plate-like glass material G lumps G _G by press-forming of the molten glass was made.

Similar to the first modification, the apparatus 101 of this modification includes a gob forming mold 180. Further, unlike the embodiment described above, the apparatus 101 according to the present modification does not include the cutting unit 160.
As shown in FIG. 8 (a), molten glass material L _G is continuously flowing out from the molten glass outflow port 111. By closing the block 181 and 182 along the path of the molten glass _{L G,} mass is cut by the cutting unit 160 molten glass _{L G} is received by the recess 180c.

Next, as shown in FIG. 8B, the blocks 181 and 182 are quickly moved vertically downward at a predetermined timing. Thus, the molten glass L _G is cut. Cut molten glass is received in a gob forming mold 180, the gob G _G Summary spherical by the surface tension.
Next, as shown in FIG. 8 (c), by the block 181, 182 is opened, the gob _{G G} formed in the recess 180c falls vertically downward.

In this modification, at the timing when the gob G _G enters the first die 121 in the gap of the second die 122, the first die 121 in a direction gob G _G drops the second die 122 moves thereby, the first die 121 and second die 122 is so as to sandwich the gob _{G G,} a first driving unit 123 and the second driving unit 124 is driven. Specifically, the first mold 121 moves in the direction of the arrow indicated by A in FIG. The second mold 122 moves in the direction of the arrow indicated by B in FIG. Accordingly, as shown in FIG. 8D, the gob _GG is captured (caught) between the first mold 121 and the second mold 122.

According to this modified example, the gob can be formed without using the cutting unit 160, and therefore, the cut mark by the cutting unit 160 is not formed on the gob. Therefore, the sheer mark resulting from the cut mark is not formed on the sheet glass material.
Further, while moving the pressing unit in the direction of the gob G _G drops pressing unit by sandwiching the gob G _G, the relative speed of the gob G _G against the press unit is decreased. As a result, the roundness of the sheet glass material G is improved, and the occurrence of cracks in the grinding process and the polishing process can be suppressed.

(Modification 3)
Next, the manufacturing method of the glass substrate for magnetic disks of the modification 3 is demonstrated. The manufacturing method of the glass substrate for magnetic disks of this modification differs from the embodiment described above in the press molding process. Since other processes are the same as those in the above-described embodiment, description thereof is omitted. Hereinafter, with reference to FIG. 9, the press molding process of this modification is demonstrated. In this modification, pressing the mass C _P of the optical glass heated at the softening furnace (not shown) instead of the gob G _G. FIG. 9A is a side view before dropping the lump of optical glass. FIG. 9B is a side view of a state in which a lump of optical glass is falling. FIG. 9C is a side view of a state in which the press unit presses a lump of optical glass.

The apparatus 101 according to this modification includes a glass gripping mechanism 112. As shown in FIG. 9 (a), a glass holding mechanism 112, the chunk C _P of the optical glass heated at the softening furnace gripped, moved to the top of the press unit 120.
As shown in FIG. 9 (b), the glass gripping mechanism 112 releases the gripping of the mass C _P of the optical glass. Thus, mass C _P of the optical glass drops vertically downward.

In this modification, at the timing when the lump C _P of the optical glass enters the first die 121 in the gap of the second die 122, the first die 121 in a direction lump C _P of the optical glass drops second with the die 122 is moved, the first die 121 and second die 122 is so as to sandwich the mass _{C P} of the optical glass, a first driving unit 123 and the second driving unit 124 is driven. Specifically, the first mold 121 moves in the direction of the arrow indicated by A in FIG. Further, the second mold 122 moves in the direction of the arrow indicated by B in FIG. 9B. As a result, as shown in FIG. 9C, the optical glass lump _CP is captured (caught) between the first mold 121 and the second mold 122.

As in this modified example, in the case of pressing the mass C _P of the optical glass heated at the softening furnace also lumps of the press unit is optical glass while moving the pressing unit in the direction of mass C _P of the optical glass falls C by sandwiching the _P, the relative speed of the mass C _P of the optical glass is reduced with respect to the press unit. As a result, the roundness of the sheet glass material G is improved, and the occurrence of cracks in the grinding process and the polishing process can be suppressed.

  Hereinafter, the results of simulations and experiments for confirming the effects of the present invention will be described using examples and comparative examples.

(Example 1-9, Comparative Example 1-9)
A plate-shaped glass material G was created by the press forming process of the first embodiment described with reference to FIG. With dropping the gob G _G heated at the softening furnace vertically downward, and press the gob G _G moves the press unit vertically downward press immediately before. Press to immediately before the gob G _G drops axis velocity of V _G of _the speed of the press units falling direction and V _M, Table 1, respectively varied V _G, V _M a as shown in Table 2 the gob G _G was measured roundness of sheet glass material G at the time of press-Te. The measured roundness is as shown in Tables 1 and 2.

As shown in Table 2, in Comparative Example 1-9, the relative speed between the gob _GG and the press unit is 2 m / s or more immediately before pressing, and the roundness of the sheet glass material G increases. In particular, as the relative velocity of the gob G _G and the press unit is increased, also it can be seen significantly roundness.
On the other hand, as shown in Table 1, in each of Examples 1-9, the press unit was moved vertically downward immediately before pressing, and the relative speed between the gob _GG and the press unit was 1 m / s or less. Therefore, the roundness of the sheet glass material G is smaller than that of Comparative Example 1-9. That is, according to Example 1-9, the sheet glass material G excellent in roundness can be produced.

(Examples 1, 10-12, Comparative Examples 1, 10, 11)
A plate-shaped glass material G was created by the press forming process of the first embodiment described with reference to FIG. Each example, in each of Comparative Examples, by varying the temperature of the gob G _G, with different viscosity of the gob G _G.
Press to immediately before the gob G _G drops axis velocity V _G of _the velocity V _M of the press unit of the dropping _direction, the viscosity of the gob G _G, roundness measured is as shown in Table 3.

As shown in Table 3, in Comparative Examples 1, 10, 11, as the viscosity of the gob G _G becomes lower, the circularity of the sheet glass material G increases. In contrast, in Examples 1,10-12, even if low viscosity of the gob G _G, roundness of the plate-like glass material G is not increased. Thus, by moving the press unit vertically downward immediately before pressing and setting the relative speed between the gob _GG and the press unit to 0, for example, the gob G having a low viscosity such as 500 poise to 700 poise is used. _{Even when G} is press-molded, a sheet glass material G having excellent roundness can be produced.

  As mentioned above, although the manufacturing method of the glass substrate for magnetic discs of this invention was demonstrated in detail, this invention is not limited to the said embodiment and modification. It goes without saying that various improvements and modifications may be made without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 1 Magnetic disk 2 Glass substrate 3A, 3B Magnetic layer 4A, 4B Magnetic head 5 Outer peripheral edge part 101 Apparatus 111 Molten glass outlet 112 Glass holding mechanism 120, 130, 140, 150 Press unit 121 1st type | mold 121a, 122a Inner circumference Surface 122 Second mold 122b Spacer 123 First driving unit 124 Second driving unit 160 Cutting unit 161 First cutting blade 162 Second cutting blade 171 First conveyor 172 Second conveyor 173 Third conveyor 174 Fourth conveyor 180 Gob formation Mold 180c Recess 181 and 182 Block 402 Lower surface plate 404 Upper surface plate 406 Internal gear 408 Carrier 409 Gear 410 Diamond sheet 412 Sun gear 414 Internal gear 416 Container 418 Coolant 420 Pump 422 filter

Claims (6)

A method for producing a glass substrate for a magnetic disk having a pair of main surfaces,
A dropping process for dropping a molten glass lump;
From both sides of the lump dropping path, a pressing step of forming a sheet glass material by sandwiching and pressing the lump at the same timing on the surfaces of a pair of molds facing each other;
A processing step of processing the sheet glass material,
For the velocity component of the lump falling direction, the pressing step is performed so that the pair of molds moves in the direction in which the lump falls, so that the relative speed of the lump with respect to the pair of molds approaches 0. A method for producing a glass substrate for a magnetic disk, characterized by being sandwiched.   2. The method of manufacturing a glass substrate for a magnetic disk according to claim 1, wherein a relative velocity of the mass with respect to the pair of molds is set to 0 with respect to a velocity component in a falling direction of the mass.   The method of manufacturing a glass substrate for a magnetic disk according to claim 1, wherein the temperature of the pair of molds is a temperature equal to or lower than a strain point of the glass. When the glass is converted to oxide standards,
The SiO ₂ 50~75%,
1 to 15% of Al ₂ O ₃ ,
A total of 5 to 35% of at least one component selected from Li ₂ O, Na ₂ O and K ₂ O;
0 to 20% in total of at least one component selected from MgO, CaO, SrO, BaO and ZnO, and
0 to 10% in total of at least one component selected from ZrO ₂ , TiO ₂ , La ₂ O ₃ , Y ₂ O ₃ , Ta ₂ O ₅ , Nb ₂ O ₅ and HfO ₂ ,
The manufacturing method of the glass substrate for magnetic discs in any one of Claims 1 thru | or 3 which consists of a composition which has.   The manufacturing method of the glass substrate for magnetic discs in any one of Claims 1 thru | or 4 whose viscosity of the said fuse | melted glass exists in the range of 500 poise -1050 poise. The processing step is
A grinding step of grinding the main surface using fixed abrasive grains;
After the grinding step, a polishing step for polishing the main surface using loose abrasive grains,
The manufacturing method of the glass substrate for magnetic discs in any one of Claims 1 thru | or 5 which has these.