JP6962702B2 - Molds for press molding, manufacturing methods for glass blanks, and manufacturing methods for glass substrates for magnetic disks - Google Patents

Description

The present invention relates to a mold for press molding, a method for manufacturing a glass blank, and a method for manufacturing a glass substrate for a magnetic disk.

Today, personal computers, DVD (Digital Versatile Disc) recording devices, and the like have a built-in hard disk drive (HDD) for data recording. In a hard disk device, a magnetic disk provided with a magnetic layer on a substrate is used, and magnetic recording information is recorded or read on the magnetic layer by a magnetic head slightly levitated on the surface of the magnetic disk. As the substrate of this magnetic disk, a glass substrate having a property of being less likely to be plastically deformed than a metal substrate (aluminum substrate) or the like is preferably used.

A glass substrate for a magnetic disk is manufactured by machining a glass blank such as grinding and polishing. As one of the methods for producing a glass blank, a method of press-molding a block of molten glass with a pair of press-molding molds is known.

In addition, a plurality of lower molds are arranged in a ring shape on the turntable, a molten glass supply unit is placed on the upper part of one lower mold, and the upper mold is placed on the upper part of the other lower mold adjacent to the lower mold. , The molten glass ingots are sequentially placed on the lower mold, the turntable is rotated, the lower mold that receives the molten glass ingot is moved below the upper mold and press-molded, and then the turntable is rotated. There is also a method of cooling the molded glass blank on the lower mold (see, for example, Patent Document 1).

A glass substrate for a magnetic disk is manufactured by performing processing such as a circular hole forming treatment, a grinding treatment, and a polishing treatment on the glass blank thus manufactured. A magnetic disk can be obtained by forming a magnetic film on the main surface of the obtained glass substrate for a magnetic disk.

International Publication No. 2013/001723

It has been found that in the method of Patent Document 1, the surface texture of the glass blank may deteriorate as the press molding is repeated. Since the main surface of a glass substrate for a magnetic disk is required to have a high surface texture, a glass blank having a deteriorated surface texture has a problem that a large amount of allowance is required for grinding and polishing treatments, resulting in a decrease in productivity. ..

An object of the present invention is to provide a press molding mold capable of accurately molding a glass blank having a good surface texture. Another object of the present invention is to provide a method for manufacturing a glass blank and a method for manufacturing a glass substrate for a magnetic disk using such a press molding mold.

The present inventor has found that as the press molding process is repeated, cracks occur on the surface of the mold with which the molten glass comes into contact, and the surface properties of the glass blank deteriorate due to this. Then, they have found that by performing a press molding process using a mold made of a specific material, cracks in the mold can be suppressed and a glass blank having a good surface texture can be molded with high accuracy, and the present invention has been completed.

One aspect of the present invention is a press molding process mold in which a molten glass is placed and the molten glass is sandwiched between the molten glass and a first mold to form a second mold for forming a glass blank.
The press molding processing type, Ri thermal expansion coefficient of 1.0~6.0 × 10 ^-6 / ℃ der in the temperature range of 350 to 1000 ° C.,
The press molding process mold is characterized by containing silicon nitride having a β-type crystal structure in an amount of 80% by mass or more of the entire press molding process mold.

The press molding process mold preferably has a thermal conductivity of 10 to 80 W / m · K in the temperature range of 350 to 1000 ° C.

The surface of the press molding process mold on which the molten glass is placed preferably has a flatness of 10 μm or less and an arithmetic mean roughness Ra of 0.5 to 5 μm.

The fracture toughness value of the press molding process mold ^{is preferably 4 MPa · m 1/2} or more.

Electrical resistivity of type for the press molding process, is preferably 1.0 × 10 ⁴ Ω · cm or more.

The press molding process mold preferably has a coefficient of thermal expansion of 4.0 to 6.0 × 10 ^-6 / ° C. in the temperature range of 350 to 1000 ° C.

The material constituting the press molding process mold contains metal nitride as a main component.
It is preferable that an oxide film obtained by oxidizing the metal nitride is arranged in at least a central region of the surface of the press molding process mold facing the first mold.

Another aspect of the present invention is a method for producing a glass blank.
Molten glass, comprising a first mold, and press-molding process for mold constituting the second type of the molten glass is placed, a press-molding process for molding the glass blank is sandwiched between,
The press molding process ^{mold has a coefficient of thermal expansion of 1.0 to 6} × 10 -6 / ° C. in a temperature range of 350 to 1000 ° C.
The press molding process mold is characterized by containing silicon nitride having a β-type crystal structure in an amount of 80% by mass or more of the entire press molding process mold.
The press molding process mold preferably has a coefficient of thermal expansion of 4.0 to 6.0 × 10 ^-6 / ° C. in the temperature range of 350 to 1000 ° C.

Another aspect of the present invention is a method for manufacturing a glass substrate for a magnetic disk.
It is characterized by having a polishing treatment in which the main surface of the glass blank produced by the method for producing a glass blank is at least polished to make the main surface a mirror surface having an arithmetic mean roughness Ra of 0.2 nm or less. ..

According to the present invention, cracks in the mold due to the press molding process can be suppressed, and a glass blank having good surface properties and a glass substrate for a magnetic disk can be molded with high accuracy.

It is a perspective view which shows the press molding processing apparatus. It is a figure which shows typically the lower part and the lower die of the press machine of the press molding processing apparatus of FIG. It is a graph which shows the relationship between the thermal expansion coefficient of the lower mold used in Example 7 and Comparative Example 3 and temperature.

Hereinafter, a method for manufacturing a glass substrate for a magnetic disk, a method for manufacturing a glass blank, and a mold for press molding according to the embodiment of the present invention will be described in detail. The present invention is a glass substrate for a magnetic disk having a nominal size of 2.5 to 3.5 inches (diameter 65 to 95 mm) and a plate thickness of 0.1 to 1.5 mm, preferably a plate thickness of 0.3 to 0.9 mm. Suitable for the production of.

(Glass substrate for magnetic disks)
First, a glass substrate for a magnetic disk will be described. The glass substrate for a magnetic disk has a disk shape. The glass substrate for a magnetic disk may have a ring shape in which a circular center hole concentric with the outer circumference is hollowed out. A magnetic disk is formed by forming a magnetic layer (recording area) in an annular region on both sides of a glass substrate for a magnetic disk.

(Glass blank for magnetic disks)
The glass blank for a magnetic disk (hereinafter, simply referred to as a glass blank) is a glass plate produced by a press molding process, and is before the grinding process described later is performed. The shape of the glass blank is substantially circular. Here, the "substantially circular" includes a perfect circle shape and an elliptical shape, and the outer peripheral shape thereof may consist of only arcs having a single radius of curvature or arcs having different radii of curvature. It may be. Further, the glass blank may have circular holes formed by the circular hole forming treatment described later.
As a material for the glass blank, aluminosilicate glass, soda-lime glass, borosilicate glass and the like can be used. In particular, aluminosilicate glass can be preferably used in that it can be chemically strengthened and that a glass substrate for a magnetic disk having excellent flatness of the main surface and strength of the substrate can be produced.

(Manufacturing method of glass substrate)
Next, a method of manufacturing a glass substrate for a magnetic disk will be described. First, a glass blank is produced by press-molding a molten glass block which is a material for a plate-shaped glass substrate for a magnetic disk having a pair of main surfaces (press molding process). Next, a circular hole is formed in the central portion of the produced glass blank to form a ring shape (annular ring) (circular hole forming treatment). Among these processes, a method of producing a glass blank by performing at least a press molding process is particularly referred to as a method for producing a glass blank. The method for manufacturing a glass blank is included in the method for manufacturing a glass substrate for a magnetic disk.
Next, the glass blank is subjected to shape processing by end face grinding (shape processing). As a result, a ring-shaped (annular) glass substrate is generated. Next, the main surface is ground with fixed abrasive grains (grinding treatment), and the end face is polished on the flat glass substrate (end face polishing treatment). Next, the main surface of the glass substrate is first polished (first polishing treatment). Next, if necessary, the glass substrate is chemically strengthened (chemical strengthening treatment). Next, the chemically strengthened glass substrate is subjected to the second polishing (second polishing treatment). Through the above processing, a glass substrate for a magnetic disk can be obtained. Hereinafter, each process will be described in detail.

(A) Press molding process The molten glass stream is cut by a cutting device, the cut molten glass ingot is sandwiched between the pressed surfaces of a pair of molds, and pressed to form a glass blank. In the present embodiment, as will be described later, after the tip of the molten glass flow is dropped on the upper surface of the lower mold, the molten glass flow is cut at a position on the upstream side to form a lump of molten glass, and the molten glass is formed. A glass blank is formed by pressing the mass downward from above with an upper mold.
After pressing for a predetermined time, the mold is opened and the glass blank is taken out.

(B) Circular hole forming treatment A disk-shaped glass blank having circular holes is obtained by forming circular holes (circular holes) in the glass blank by coring, scribing, or the like.

For coring, a glass blank is cut from one main surface with a cylindrical core drill with one end open, and the circumference of the circular hole is scraped off to hollow out the glass at the center (core) to form a through hole. How to do it. The circumference of the circular hole (inner circle) may be scraped off, and the circular cutting line (outer circle) serving as the outer contour line of the glass blank may be scraped off by a core drill. Then, the portion outside the outer circle and the portion inside the inner circle of the glass blank are removed to obtain a disk-shaped glass blank.

For scribing, a circular cutting line is provided on one main surface of the glass blank by a cutter (scriber) made of cemented carbide or diamond particles, and then the circular cutting line is formed by heating the glass blank to obtain the thickness of the glass blank. This is a method of extending in a direction and pressing the inside of a circular cutting line to separate the glass. At the same time as the circular cutting line that serves as the contour line of the circular hole, the circular cutting line that serves as the outer contour line of the glass blank may be formed at the same time. In this case, the circular cutting line (outer circle) that is the outer contour line of the glass blank and the circular cutting line (inner circle) that is the contour line of the circular hole are formed so as to be concentric. Then, by partially heating the glass blank, the part outside the outer circle and the part inside the inner circle of the glass blank are removed due to the difference in thermal expansion of the glass blank, and the disk-shaped glass blank is formed. can get.

(C) Shape processing In the shape processing, chamfering is performed on the outer peripheral edge of the glass blank. When a circular hole is formed in the glass blank, chamfering is also performed on the inner peripheral end of the circular hole.

(D) Grinding process In the grinding process, a double-sided grinding device equipped with a planetary gear mechanism is used to grind the main surface of the glass blank. Specifically, both main surfaces of the glass blank are ground while holding the outer peripheral end faces of the glass blank in the holding holes provided in the holding member of the double-sided grinding device. The double-sided grinding device has a pair of upper and lower surface plates (upper surface plate and lower surface plate), and a glass substrate is sandwiched between the upper surface plate and the lower surface plate. Then, by moving one or both of the upper surface plate and the lower surface plate and relatively moving the glass blank and each surface plate, both main surfaces of the glass blank can be ground. Thereby, the plate thickness can be adjusted and the flatness can be further improved.

(E) End face polishing treatment In the end face polishing treatment, the outer peripheral side end face of the glass blank is mirror-finished by brush polishing. When a circular hole is formed in the glass blank, the inner peripheral end surface of the circular hole is also mirror-finished. At this time, an abrasive grain slurry containing fine particles such as cerium oxide as free abrasive grains is used.

(F) First polishing treatment In the first polishing treatment, for example, when grinding with fixed abrasive grains is performed, scratches and distortions remaining on the main surface are removed, or minute surface irregularities (microwaveness, roughness) are removed. For the purpose of adjustment. Specifically, the main surfaces on both sides of the glass substrate can be polished while holding the outer peripheral end faces of the glass substrate obtained by grinding the glass blank in the holding holes provided in the polishing carrier of the double-sided polishing apparatus. Will be done.

In the first polishing process, the glass substrate is polished while applying a polishing slurry by using a double-sided polishing device having the same configuration as the double-sided grinding device used for the grinding process using fixed abrasive grains. In the first polishing process, unlike grinding with fixed abrasive grains, a polishing slurry containing free abrasive grains is used instead of the fixed abrasive grains.

Like the double-sided grinding device, the double-sided polishing device has a pair of upper and lower surface plates (upper surface plate and lower surface plate), and a glass substrate is sandwiched between the upper surface plate and the lower surface plate. An annular flat plate polishing pad (for example, a resin polisher) is attached to the upper surface of the lower surface plate and the bottom surface of the upper surface plate as a whole. By moving one or both of the upper surface plate and the lower surface plate, the glass substrate and each surface plate are relatively moved, so that both main surfaces of the glass substrate are polished.

The glass substrate may be chemically strengthened by immersing the glass substrate after the first polishing treatment in the chemical strengthening liquid (chemical strengthening treatment). As the chemical strengthening liquid, for example, a mixed melt of potassium nitrate and sodium sulfate can be used.

(G) Second polishing (final polishing) treatment The second polishing treatment aims at mirror polishing of the main surface. Also in the second polishing, a double-sided polishing apparatus having the same configuration as the double-sided polishing apparatus used for the first polishing is used. Specifically, the main surfaces on both sides of the glass substrate are polished while holding the outer peripheral end faces of the glass substrate in the holding holes provided in the polishing carrier of the double-sided polishing apparatus. The second polishing treatment differs from the first polishing treatment in that the types and particle sizes of the free abrasive grains are different and the hardness of the resin polisher is different. Specifically, a polishing liquid containing colloidal silica having a particle size of about 5 to 100 nm as free abrasive grains is supplied between the polishing pad of the double-sided polishing apparatus and the main surface of the glass substrate, and the main surface of the glass substrate is polished. NS. By cleaning the polished glass substrate with a neutral detergent, pure water, isopropyl alcohol, or the like, a glass substrate for a magnetic disk can be obtained.

Here, the press molding processing apparatus used for the press molding processing will be described. FIG. 1 is a perspective view of a press molding processing apparatus used for press molding processing of a glass blank.
As shown in FIG. 1, the press forming processing apparatus includes a turntable 11, a plurality of lower press machines 12, a plurality of lower molds (second molds) 20, and upper molds (first molds) 30. A press machine upper portion 13, a rotating shaft 14, and an outflow nozzle 15 are provided.

The turntable 11 has a disk shape, and a plurality of lower presses 12 are fixed to the upper part of the turntable 11 in a state of being arranged at equal intervals in the circumferential direction. Lower molds 20 are fixed to the upper portions of the lower portions 12 of the plurality of press machines. The lower mold 20 will be described in detail later.
A rotating shaft 14 is provided at the center of the turntable 11. The turntable 11 falls on the lower portion 12 of the press machine, the plurality of lower dies 20, and the upper surface (press surface 21) of the lower dies 20, and together with the molten glass placed and the formed glass blank, the rotating shaft 14 is formed. Rotate to the center.
A lower mold 20 is provided on the upper portion of the lower portion 12 of the press machine. Further, a heater (not shown) for controlling the temperature of the lower mold 20 is embedded inside the lower portion 12 of the press machine.

An outflow nozzle 15 is provided above any one of the plurality of lower molds 20. The outflow nozzle 15 causes the molten glass 16 to flow out onto the upper surface of the lower mold 20 arranged below the outflow nozzle 15. The molten glass 16 that has fallen on the upper surface is cut by a blade (not shown) at a position near the outlet of the outflow nozzle 15, and is placed on the upper surface of the lower mold 20 as a molten glass block 17. The cut marks of the molten glass cut by the blade are preferably placed in the center of the glass blank by a press forming process so as to be removed by a circular hole forming process which is a later step.

Further, among the plurality of lower dies 20, a press upper portion 13 is provided above the lower dies 20 arranged on the downstream side in the rotation direction of the turntable 11 with respect to the one in which the outflow nozzle 15 is arranged above. ing. An upper mold 30 is provided below the upper portion 13 of the press machine. Further, a heater (not shown) for controlling the temperature of the upper die 30 is embedded inside the upper portion 13 of the press machine.
Each time the lower 12 of the press supporting the lower mold 20 on which the molten glass block 17 is placed is transferred below the upper 13 of the press by the turntable 11, the upper 13 of the press is controlled by the servomotor to 30 the upper mold. By lowering the molten glass block 17 by a predetermined distance, the molten glass block 17 is sandwiched between the upper mold 30 and the lower mold 20 at a predetermined interval, the molten glass block 17 is stretched to a predetermined plate thickness, and then the molten glass block 17 is raised. repeat.

The upper mold 30 presses the molten glass block 17 placed on the upper surface of the lower mold 20 downward. The upper mold 30 is made of a material having higher thermal conductivity than the lower mold 20 (for example, a tungsten alloy). Therefore, the molten glass ingot 17 placed on the upper surface of the lower mold 20 is maintained in a high temperature state until it comes into contact with the upper mold 30, and the upper mold 30 presses the molten glass ingot 17 from above the molten glass ingot 17. By doing so, the molten glass block 17 in contact with the upper mold 30 is rapidly cooled and formed into a glass blank 18.
The molded glass blank 18 is conveyed by the turntable 11 while being cooled while being placed on the upper surface of the lower mold 20. The cooled glass blank 18 is removed from the upper surface of the lower mold 20 by a suction means (not shown), and is conveyed to an apparatus for performing subsequent processing such as shape processing.

(Mold for press molding)
Next, the lower mold 20 of the present embodiment will be described in detail.
The lower mold 20 is made of a sintered material having a ^{coefficient of thermal expansion of 1.0 to 6.0 × 10 -6} / ° C. in the temperature range of 350 to 1000 ° C. When the lower mold 20 is made of such a material, the surface texture of the molded glass blank becomes good as described below.
When the high-temperature molten glass comes into contact with the lower mold press surface, a temperature difference occurs between the surface (lower surface) on the opposite side of the press surface or between the central portion and the outer peripheral portion of the press surface, resulting in heat. Differences in the amount of expansion are likely to occur. Therefore, along with the press molding process, the high-temperature molten glass comes into contact with each other at a high pressure, and thermal expansion and contraction are repeated, and as a result, cracks are likely to occur on the pressed surface. Since the cracked press surface has irregularities, the surface texture of the press surface is transferred to the surface of the glass blank, and the surface texture of the main surface (lower main surface) of the glass blank that comes into contact with the lower mold is changed. Easy to make worse. In particular, since the lower mold repeats thermal expansion and contraction in the surface direction of the press surface, cracks move along the surface direction of the lower mold, which tends to deteriorate the surface texture of the glass blank.
In addition, the cracks generated on the pressed surface may further progress to become larger cracks as the press molding process is continuously performed, leading to damage to the lower mold. In particular, the mold used for molding the glass blank, which is the base plate of the glass substrate for a magnetic disk, has a larger heat capacity of the molten glass block than the molding mold for an optical lens or the like, so that cracks and breakage are likely to occur. Therefore, the accuracy of molding the glass blank drops in a short period of time. In addition, the lower mold, which has a short life, is frequently replaced, which increases the production cost.
Since the lower mold 20 of the present embodiment is made of a material having a coefficient of thermal expansion satisfying the above range, cracks due to the press molding process are unlikely to occur. Therefore, a glass blank having a good surface texture can be molded with high accuracy. In particular, since cracks are likely to occur in the central portion of the press surface 21 which becomes hot during the press molding process, unevenness is more likely to occur in the central portion than in the outer peripheral portion, and due to this, the lower side of the molded glass blank. The properties of the main surface tend to be uneven. However, since the lower mold 20 is suppressed from cracking, the surface texture of the pressed surface is unlikely to be non-uniform between the central portion and the outer peripheral portion, and the texture of the lower main surface of the molded glass blank is maintained uniformly. Easy to be done.

In the present specification, the coefficient of thermal expansion is the coefficient of linear expansion measured by thermomechanical analysis in accordance with JIS R1618: 2002. The temperature range of 350 to 1000 ° C. is the temperature range of the lower mold 20, and when a temperature distribution occurs in the lower mold 20, it means the temperature range of the portion showing the maximum temperature of the lower mold 20. The above temperature range includes a range (for example, 350 to 700 ° C.) of a temperature (molding temperature) preset as the temperature of the lower mold 20 in the press molding process, and the molding temperature is caused by contact with the high-temperature molten glass. Also includes the temperature of the lower mold 20 when it temporarily exceeds.
A material having a coefficient of thermal expansion that is temperature-dependent may vary significantly from the coefficient of thermal expansion at room temperature, for example, in the above temperature range. According to the present inventor, when a material having a coefficient of thermal expansion of 1.0 to 6.0 × 10 ^-6 / ° C. is used for the lower mold 20, cracks are less likely to occur and the surface texture of the glass blank is improved. It was found that the deterioration of the disease can be suppressed. If the coefficient of thermal expansion ^{exceeds 6.0 × 10 -6} / ° C, the amount of thermal expansion of the lower mold 20 during the press molding process is large, and the occurrence of cracks cannot be suppressed. If the coefficient of thermal expansion is ^{less than 1.0 × 10 -6} / ° C, the amount of thermal expansion with respect to the lower portion 1 of the press machine is too small, and cracks may occur on the lower surface of the lower mold 20.
The coefficient of thermal expansion of the material of the lower mold 20 is preferably 3.0 to 5.5 × 10 ^-6 / ° C. in the temperature range of 800 to 1000 ° C. When the coefficient of thermal expansion of the lower mold 20 is within such a range, the effect of suppressing the occurrence of cracks in the lower mold 20 is enhanced, and the deterioration of the surface texture of the glass blank is likely to be suppressed. More preferably, the coefficient of thermal expansion of the material of the lower mold 20 is such that the average value of the coefficient of thermal expansion in each range of 800 to 810 ° C., 970 to 980 ° C., 1030 to 1040 ° C. is 3.0 × 10 ^-6 / It is in the range of ° C or higher and 4.0 × 10 ^{-6 / ° C or lower.} When the coefficient of thermal expansion of the material of the lower mold 20 ^{is 3.0 to 5.5 × 10 -6} / ° C in the temperature range of 800 to 1000 ° C., and the above average value is 3.0 × 10 ^-6 / ° C. When the temperature ^{is within the range of 4.0 × 10 -6} / ° C or lower, when the press molding process is continuously repeated, cracks and dents due to shedding occur for 1 week or longer, preferably 10 days or longer. The occurrence can be suppressed and the durability is high.

The material of the lower mold 20 further preferably has a thermal conductivity of 10 to 80 W / m · K in the temperature range of 350 to 1000 ° C. Further, it is more preferable that the material of the lower mold 20 has a thermal conductivity of 10 to 80 W / m · K in the temperature range of 250 to 1000 ° C. In the present specification, the thermal conductivity means a value calculated in accordance with JIS R1611: 2010. Thermal conductivity generally decreases at higher temperatures. The present inventor continuously supplies a part of the lower mold 20 with high-temperature molten glass when a material having a thermal conductivity of 10 to 80 W / m · K is used for the lower mold 20 in the above temperature range. Even if this is done, the heat transferred to the lower mold 20 is not maintained, and the heat is efficiently released and cooled, so that the sintering aid melts at the position where the molten glass of the lower mold 20 is supplied. However, as a result, it was found that the bleeding of the lower mold 20 can be suitably prevented. Since molten glass is placed on the lower mold 20 in a high temperature state, if a sintering aid (binder) having a low melting point is used, a part of the lower mold 20 adheres to the molten glass and the lower mold 20 is placed. It is easy for shedding to break away from the glass. However, when the thermal conductivity of the material of the lower mold 20 is within the above range, the shedding of the lower mold 20 is suppressed, and the pressed surface 21 of the lower mold 20 is uneven over time due to the shedding. The surface texture and flatness of the glass blank to be molded are preferably maintained. If the thermal conductivity is less than 10 W / m · K, the heat received from the molten glass is difficult to be dissipated from the lower mold 20, the sintering aid melts, and the lower mold 20 is likely to be shed. If the thermal conductivity exceeds 80 W / m · K, the molten glass will be cooled in a short time during molding, and it will be difficult to mold the glass blank with high accuracy in terms of flatness, etc., while the lower mold 20 If the temperature is raised to improve the moldability, the molten glass is fused to the lower mold 20, and as a result, the glass blank cannot be molded with a desired accuracy. Therefore, the thermal conductivity of the material of the lower mold 20 is preferably 10 to 80 W / m · K in the temperature range of 350 to 1000 ° C.

The fracture toughness value of the material of the lower mold 20 ^{is preferably 4 MPa · m 1/2} or more. In the present specification, the fracture toughness value is a value measured according to JIS R1617: 2010. When the fracture toughness value is within the above range, cracking of the lower mold 20 under high temperature and high pressure is prevented during the press molding process, and the molten glass is fused to the lower mold 20 to form the press surface of the lower mold 20. The portion forming the 21 is suppressed from being peeled off, and the press surface 21 of the lower mold 20 is suppressed from being dented. This makes it possible to form a glass blank having a high flatness (for example, a plate thickness deviation of 15 μm or less) for a long period of time, and in particular, it is possible to reduce the frequency of replacement of the lower mold 20. If the fracture toughness value is ^{less than 4 MPa · m 1/2} , when the molten glass is fused to the lower mold 20, it may not be possible to suppress the cracking of the lower mold 20 under high temperature and high pressure, and the lower mold 20 may not be suppressed. The portion forming the press surface 21 of the above is easily peeled off, and the press surface 21 may be dented over time due to repeated press molding processes. Such cracks and dents on the pressed surface 21 deteriorate the flatness of the lower mold 20, and accordingly, the flatness of the lower main surface of the glass blank. The upper limit of the fracture toughness value of the lower mold 20 is not particularly limited, but is, for example, 20 MPa · m ^1/2 .

Electrical resistivity of the material of the lower die 20 is preferably 1.0 × 10 ⁴ Ω · cm or more. The outflow nozzle 15 of the press forming processing apparatus may energize and heat the molten glass by supplying an electric current and generating heat in order to maintain the temperature of the molten glass. However, when the electrical resistivity of the lower mold 20 ^{is less than 1.0 × 10 4} Ω · cm, the outflow nozzle 15 and the lower mold 20 conduct with each other through the molten glass, so that the molten glass is heated and bubbles are generated. May occur. When such bubbles are generated, they remain in the glass blank, and in the process from the shape processing process to the second polishing process, recesses are formed on the surface of the glass blank, or a part of the glass surrounding the bubbles. May come off and adhere to the surface of the glass blank, contaminating it. When the electrical resistivity of the material of the lower mold 20 is within the above range, it is possible to prevent the outflow nozzle 15 and the lower mold 20 from being electrically connected to each other through the molten glass that has fallen on the press surface 21. Metals are not included in the materials whose electrical resistivity is in the above range. The upper limit of the electrical resistivity is not particularly limited.

The material of the lower mold 20 is a material satisfying the above thermal expansion coefficient, for example, a metal nitride, a metal oxide, a metal carbide, a metal boride, a blend of two or more of these, or a material thereof. Examples include those blended with other substances. Among them, those containing a metal nitride are preferably used in that a lower mold 20 having high durability can be obtained. The metal constituting the metal compound contains elements such as boron and silicon, which are called metalloids. The other substances include a sintering aid (binder) when the lower mold 20 is a powder sintered body, and a substance (glassy) which becomes a glass state by sintering. In addition to the coefficient of thermal expansion, the material of the lower mold 20 preferably has one or more of thermal conductivity, fracture toughness value, and electrical resistivity satisfying the above ranges. If the sintering aid has a low melting point, it may cause shedding when the molten glass comes into contact with the lower mold 20, so the sintering aid having a melting point of 1000 ° C. or higher is used below. It is preferable to use 30% by mass or less, preferably 15% by mass or less of the entire mold 20.
Specific examples of the material of the lower mold 20 include silicon nitride (Si ₃ N ₄ ), aluminum nitride (AlN), silicon carbide (SiC), alumina (Al ₂ O ₃ ), zirconia (ZrO ₂ ), and sialon (Si _3). N ₄ , Al ₂ O ₃ ), or a blend of these with other substances can be mentioned. Of these, silicon nitride (Si ₃ N ₄ ), aluminum nitride (Al N _{), Sialon (Si 3 N 4} · Al ₂ O ₃ ), or a blend of these with other substances is preferably used. Specific examples of the binder include oxides or nitrides of metals such as yttrium and erbium, and alloys of two or more kinds of metals such as yttrium, aluminum, iron, titanium and zirconium. Specific examples of vitreous materials include alumina, itria, and the like.
The content of metal nitride, metal oxide, metal carbide, and metal boride, which are the main components of the material of the lower mold 20, and the content of a blend of two or more of these are 80% by mass or more of the entire lower mold 20. It is preferably 90% by mass or more, and more preferably 90% by mass or more. The material of the lower mold 20 preferably contains glass, and in this case, it is preferably contained in an amount of 3 to 20% by mass of the entire lower mold 20 from the viewpoint of maintaining strength at high temperatures.
The lower mold 20 can be produced by sintering the powder of the above material, but may be produced by, for example, molding by a cold isotropic press and then degreasing and sintering. The sintering may be normal pressure firing, but it is preferably compacted by gas pressure firing or hot isostatic pressing (HIP) after sintering.

The material of the lower mold 20 contains silicon nitride having a β-type crystal structure in an amount of preferably 80% by mass or more, more preferably 100% by mass. By forming the lower mold 20 with such a material, the fracture toughness, oxidation resistance, and strength of the lower mold 20 are improved. In the above range, the material of the lower mold 20 may include silicon nitride having an α-type crystal structure as silicon nitride having a crystal structure other than the β-type crystal structure. The coefficient of thermal expansion generally changes depending on the type of material used for the lower mold, the crystal structure, and the firing method thereof. For example, when the lower mold material contains a large amount of silicon nitride having an α-type crystal structure (for example, 10% by mass or more of the entire lower mold), the value of the coefficient of thermal expansion tends to increase. In this respect, the material of the lower mold 20 is preferably made of silicon nitride having a β-type crystal structure and does not contain an α-type crystal structure.

The oxygen concentration in the elements constituting the press surface 21 of the lower mold 20 is preferably (15 atomic%) or more. Specifically, it is preferable that an oxide film obtained by oxidizing the metal nitride is arranged at least in the center of the press surface 21 of the lower mold 20. The central portion of the press surface 21 refers to a region of the distance from the center of the press surface 21 to the contour line, which is a distance of 30% to 45% from the center. If the contour of the lower mold 20 is not a perfect circle, the center of the lower mold 20 is the center of the smallest circumscribed circle that circumscribes the contour of the lower mold 20. The outer peripheral portion refers to the region of the press surface 21 excluding the central portion. Since the oxide film is arranged at the center of the press surface 21 of the lower mold 20 and the oxygen concentration is high, it becomes difficult for the molten glass to be fused to the lower mold 20. The reason for this is that an oxide film is formed on the press surface 21 by the oxidation treatment of the lower mold 20, so even if the molten glass is fused with the oxide film, the oxide film is formed when the glass blank is removed from the lower mold 20. It is considered that this is because the glass peeled from 20 and fused with the oxide film does not remain on the pressed surface 21. On the other hand, in the lower mold 20 after press molding, a new oxide film is formed by heating with molten glass, so that the proportion of the metal oxide region on the press surface 21 is kept constant. By suppressing the fusion of the glass to the lower mold 20 in this way, the life of the lower mold 20 can be extended and the production cost can be reduced. Before using such an oxide film in the press molding treatment, the lower mold 20 is subjected to an oxidation treatment in advance to oxidize a part of the metal nitride constituting the press surface 21 into a metal oxide. It is formed by. In the oxidation treatment, for example, the metal nitride is formed into the shape of the lower mold 20 and then heated at 1000 to 1200 ° C. for 10 to 12 hours in an oxygen-containing atmosphere to oxidize the metal nitride on the surface. be able to.

The upper surface of the lower mold 20 preferably has a flatness of 10 μm or less and an arithmetic mean roughness Ra of 0.5 to 5 μm. By performing the press molding process using the lower mold 20 having such an upper surface, it is possible to mold a glass blank having a good surface texture. Therefore, the removal allowance in the grinding process or the polishing process is small, and the productivity is improved. Further, when the lower mold 20 has high durability and a long life, the accuracy of press molding can be maintained for a long period of time. The flatness can be measured using a flatness measuring machine. The arithmetic mean roughness Ra referred to in the present specification is measured by a stylus type roughness meter (contact type roughness measuring machine) using a stylus (touch needle) in accordance with JIS B0601: 2001. be. The upper surface of the lower mold 20 having the flatness and the arithmetic mean roughness Ra in the above ranges can be obtained by grinding and blasting. The flatness of the upper surface of the lower mold 20 is preferably 5 μm or less. The arithmetic surface roughness Ra of the upper surface of the lower mold 20 is more preferably 2 μm or less. Even after the above oxidation treatment and, if necessary, roughening treatment and removal treatment, the arithmetic surface roughness Ra of the upper surface of the lower mold 20 is preferably 5 μm or less, preferably 2 μm or less. It is more preferable that it is.

The thickness of the lower mold 20 is preferably 20 to 40 mm. Since the lower mold 20 is fixed to the upper part of the lower portion 12 of the press machine, the heat received by the lower mold 20 from the molten glass easily moves in the thickness direction of the lower mold 20, but on the upper and lower surfaces of the lower mold 20. A temperature difference is likely to occur between the two, and a difference in the amount of thermal expansion is likely to be large. However, when the thickness of the lower mold 20 is 20 mm or more, the strength is ensured and the effect of suppressing the occurrence of cracks is improved. Further, by increasing the thickness of the lower mold 20, the heat capacity of the lower mold 20 is increased. A more preferable thickness of the lower mold 20 is 25 to 30 mm.

The shape of the lower mold 20 is preferably a disk shape. The diameter of the lower mold 20 is determined according to the target size of the glass blank, and is preferably a disk shape having a radius of 110 to 130% with respect to the radius size of the glass blank to be molded. In this case, the ratio of the diameter to the thickness of the lower mold 20 is preferably 2 to 10 times, preferably 4 to 8 times, from the viewpoint of suppressing cracks in the lower mold 20. In the press forming process of the glass blank, a lower die 20 having a larger diameter of the press surface 21 than that of the press forming process of optical glass or the like is used, and such a lower die 20 includes the central portion and the outer peripheral portion of the press surface 21. The difference in the amount of thermal expansion between them is large, and cracks are likely to occur. Further, the unevenness of the press surface 21 caused by the cracks moves along the surface direction of the lower mold 20, so that the surface texture of the glass blank is likely to be deteriorated. However, it has been found that when the ratio of the diameter to the thickness of the lower mold 20 is within the above range, the effect of suppressing cracks on the pressed surface 21 is improved.

According to the lower mold 20 of the present embodiment, since the material is composed of a material having a coefficient of thermal expansion satisfying the above range, cracks due to the press molding process are suppressed, so that a glass blank having a good surface texture can be accurately obtained. Can be molded well. When the lower mold 20 is further made of a material having a thermal conductivity satisfying the above range, the press surface of the lower mold 20 is prevented from being uneven over time due to shedding. , The surface texture and flatness of the glass blank to be molded are preferably maintained.

(Example, comparative example)
Using the lower dies of Examples and Comparative Examples shown in Tables 1 and 2, the press molding process of the above embodiment was carried out, and the properties of the press surface of the lower dies were examined.
In the press molding process, the molten aluminosilicate glass was molded by adjusting the temperature of the molding mold to be below the glass transition point, and this molding was continuously repeated to appropriately confirm the state of the lower mold. As a glass blank, a glass blank having a diameter of 75 mm and a plate thickness of 9 mm was produced. The shape of the lower mold was 25 mm in thickness and 90 mm in diameter of the pressed surface. The flatness of the pressed surface was 10 μm, and the arithmetic surface roughness Ra was 1 μm. A tungsten alloy was used for the upper mold.

The coefficient of thermal expansion, thermal conductivity, fracture toughness value, and electrical resistivity of the lower mold shown in Tables 1 and 2 were measured as described in the above-described embodiment. The coefficient of thermal expansion was set to the maximum value among the values of the coefficient of thermal expansion measured in increments of 10 ° C. in a temperature range including 350 to 1000 ° C. Here, FIG. 3 shows a graph showing the relationship between the coefficient of thermal expansion of the lower mold used in Example 7 and Comparative Example 3 and the temperature, on behalf of Examples and Comparative Examples. The graph shown in FIG. 3 is a curve representing the regression equation obtained by regressing the plot of each measured value as an approximate equation. The horizontal axis of FIG. 3 shows a part of the measurement range in increments of 10 ° C. The thermal conductivity is shown in Tables 1 and 2 in the range starting from the values at 350 ° C. and 1000 ° C.
In Tables 1 and 2, "composition of mold material" indicates a compound as a main component. The mold materials of Examples 1 to 9 and Comparative Examples 1 to 3 were all fired using a metal compound as a main component and a sintering aid of 3 to 10% by mass of the entire lower mold. Is. Further, in the mold material of Comparative Example 3, the crystal structure after sintering was composed of α-type silicon nitride and β-type silicon nitride, and α-type silicon nitride was contained in a proportion of 15% by mass.

Molding was repeated for 10 days by continuous operation of the press forming processing apparatus, and after 5 days had passed, cracks, dents, flatness, and arithmetic surface roughness Ra of the press surface of the lower mold were measured as follows. If obvious cracks or dents were formed in the lower mold before the lapse of 5 days, the molding was finished at that point and each item was evaluated.
For cracks, the presence or absence of cracks of a size that can be visually confirmed on the pressed surface of the lower mold was confirmed visually and by using a laser microscope. The case where a crack was detected was regarded as "defective", and the case where no crack was detected was regarded as "good".
For dents, the unevenness of the pressed surface is measured using a laser displacement meter, and the case where the maximum dent depth from the reference height is 10 μm or more and the dents are deflated is regarded as “defective”, and the reference height is defined as the dent. When the maximum dent depth from the above was 5 μm or more and less than 10 μm, it was regarded as “possible”, and when it was less than 5 μm, it was regarded as “good”. The reference height was the position in the thickness direction on the pressed surface of the outer peripheral edge of the lower mold.
The flatness and the arithmetic surface roughness Ra were measured as described in the above embodiment. As a result, the case where the flatness exceeds 15 μm is regarded as “poor”, the case where the flatness is 10 μm or less is regarded as “possible”, and the case where the flatness is particularly 4 μm or less is regarded as “good”. In addition, the arithmetic surface roughness Ra is measured at 5 points at equal intervals along the radial direction of the press surface, the deviation from the average value is calculated, and if the maximum deviation exceeds 1 μm, it is “defective” and 1 μm or less. The case of "OK" was defined as "OK", and the case of 0.3 μm or less was defined as "Good".
After repeating molding for 10 days by continuous operation of the press molding processing apparatus, cracks, dents, flatness, and arithmetic surface roughness Ra were measured again, and there was no characteristic corresponding to "defective" in the above evaluation criteria. The case was evaluated as having good durability (indicated by "○" in the table), and the case having one or more "defective" characteristics was evaluated as not having good durability (in the table, "". (Represented by "x").
The results are shown in Tables 1 and 2.

From the comparison of Examples 1 to 9 and Comparative Examples 1 to 3, the lower mold is composed of a material having a ^{coefficient of thermal expansion of 1.0 to 6.0 × 10 -6 / ° C in the temperature range of 350 to 1000 ° C.} It can be seen that the cracks on the pressed surface and the dents due to the shedding can be suppressed. Further, it can be seen that deterioration of the flatness and surface roughness Ra of the pressed surface can be suppressed, and uniform maintenance can be performed at the central portion and the outer peripheral portion of the pressed surface. Regarding the molded glass blank, the flatness and surface roughness Ra measured in the same manner as in the lower mold 20 were also maintained uniformly on the lower main surface.

Further, from Examples 3, 4, 6 to 9, the lower mold is further composed of a material having a thermal conductivity in the range of 10 to 80 W / m · K under high temperature conditions (350 to 1000 ° C.). Therefore, it can be seen that the durability is high and the effect of suppressing damage caused by dents due to shedding and cracks on the pressed surface is high in long-term continuous molding in a mass production process or the like. It is considered that one of the reasons why the dent of Example 4 was better than that of Example 1 was that the high fracture toughness value of the material of Example 4 contributed greatly.

From the comparison of Examples 3, 4, 6 to 9 and Examples 1 and 2, the lower mold material is a material containing silicon nitride, and the thermal conductivity in the temperature range of 350 to 1000 ° C. is 10 to 80 W / m. -It can be seen that the durability is improved when the value is K and the fracture toughness value is 4 MPa · m ^{1/2 or more.}
Further, from the comparison between Examples 3, 4, 6 to 9 and Example 5, the thermal conductivity of the lower mold in the temperature range of 350 to 1000 ° C. is 10 to 80 W / m · K, and the coefficient of thermal expansion is high. It can be seen that the durability is improved when ^{the temperature is 3.0 to 5.5 × 10 -6} / ° C in the temperature range of 800 to 1000 ° C.

Although the press molding process mold, the glass blank manufacturing method, and the glass substrate manufacturing method for the magnetic disk of the present invention have been described in detail above, the present invention is not limited to the above embodiments and examples, and the present invention is not limited to the above embodiments and examples. Of course, various improvements and changes may be made as long as the purpose is not deviated.
For example, the press molding process mold of the present invention can also be used for a press molding process for forming a glass blank other than the glass blank used as a glass substrate for a magnetic disk.

11 Turntable 12 Lower part of press machine 13 Upper part of press machine 14 Rotating shaft 15 Outflow nozzle 16 Molten glass 17 Molten glass block 18 Glass blank 20 Lower mold 21 Press surface 30 Upper mold

Claims (10)

A press molding process mold on which molten glass is placed and which constitutes a second mold for forming a glass blank by sandwiching the molten glass between the first mold and the first mold.
The press molding process mold has a coefficient of thermal expansion of 1.0 to 6.0 × 10 ^-6 / ° C in a temperature range of 350 to 1000 ° C.
The press-molding treatment mold is a press-molding treatment mold containing 80% by mass or more of silicon nitride having a β-type crystal structure as a whole. The press molding process mold according to claim 1, wherein the press molding process mold has a thermal conductivity of 10 to 80 W / m · K in a temperature range of 350 to 1000 ° C. The press molding according to claim 1 or 2, wherein the surface of the press molding process mold on which the molten glass is placed has a flatness of 10 μm or less and an arithmetic mean roughness Ra of 0.5 to 5 μm. Processing type. The press molding process mold according to any one of claims 1 to 3, wherein the fracture toughness value of the press molding process mold is 4 MPa · m ^{1/2 or more.} The press molding process mold according to any one of claims 1 to 4, wherein the electrical resistivity of the press molding process mold is 1.0 × 10 ^{4 Ω · cm or more.} The press according to any one of claims 1 to 5, wherein the press molding process mold has a coefficient of thermal expansion of 4.0 to 6.0 × 10 ^{-6 / ° C in a temperature range of 350 to 1000 ° C.} Mold for molding process. The material constituting the press molding process mold contains metal nitride as a main component.
Any one of claims 1 to 6, wherein an oxide film obtained by oxidizing the metal nitride is arranged in at least a central region of the surface of the press molding process mold facing the first mold. The press molding processing mold described in the section. A method for manufacturing a glass blank for manufacturing a glass substrate for a magnetic disk.
Molten glass, comprising a first mold, and press-molding process for mold constituting the second type of the molten glass is placed, a press-molding process for molding the glass blank is sandwiched between,
The press molding process ^{mold has a coefficient of thermal expansion of 1.0 to 6} × 10 -6 / ° C. in a temperature range of 350 to 1000 ° C.
The press molding processing mold comprises silicon nitride having a β-type crystal structure than 80 wt% of the total press-molding process for mold manufacturing method of a glass blank, characterized in that. The method for producing a glass blank according to claim 8, wherein the press forming mold has a coefficient of thermal expansion of 4.0 to 6.0 × 10 ^{-6 / ° C in a temperature range of 350 to 1000 ° C.} A polishing treatment is performed in which the main surface of the glass blank produced by the method for producing a glass blank according to claim 8 or 9 is at least polished to make the main surface a mirror surface having an arithmetic average roughness Ra of 0.2 nm or less. A method for manufacturing a glass substrate for a magnetic disk, which comprises having.

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