JPH10194760A - Thin plate glass and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To almost completely eliminate the warpage of a press-formed product after polishing both faces and obtain a final product having excellent flatness by forming a thick part at a part of a thin plate glass and receiving the force from both surfaces with the thick part. SOLUTION: A thick part 13 for receiving the pressure by polishing plates 51, 52 on both surfaces is formed on a part, e.g. the outer circumferential edge part of a thin plate glass 11. When a thin plate glass has warpage and is free from thick part on the outer circumference, a flat thin plate glass cannot be obtained since the thin plate glass is bent by the application of pressure from both surfaces with polishing plates and, even if both surfaces of the glass are polished to flat, the warpage of the thin plate glass takes place after the release of the pressure applied by the polishing plates. The pressure from both surfaces is received by the thick part 13 in the present process to enable the polishing of both surfaces without causing the bending of the thin plate glass. It is necessary to highly draw the glass plate to get a thin plate, and the drawing is carried out by applying a solid lubricant to the forming surface of the mold.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a thin plate having a thickness of, for example, about 3 mm or less, which is used as a glass substrate for an information recording medium such as a magnetic recording medium, a magneto-optical recording medium, and an optical recording medium, and as a filter for a camera. Glass-like glass and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, as a method for producing this kind of thin plate glass, there is a production method described in, for example, JP-A-7-133121. This method is
By setting the surface temperature of the press surface of the lower die near the glass transition point and setting the inner surface temperature of the body die higher than the surface temperature of the press surface, the thin plate shape is not hindered by elongating the glass. This is a method of press forming glass.

[0003] As with this method, conventionally, when thin-walled glass is generally press-formed, a glass gob is pressed with upper and lower dies to form the thin-walled glass, and then the upper die is immediately pressed into the thin-walled glass. Although separated from the glass, the thin glass plate is held on the press surface of the lower die until the lower die moves to the glass unloading position, and is removed from the press surface of the lower die when the lower die moves to the unloading position. To the annealing step.

Therefore, when considering the heat radiation of the thin glass sheet after press forming, the upper mold side of the thin glass sheet has a poor heat radiation as a result of the upper mold being separated immediately, but the lower surface side of the thin glass sheet is lowered to the unloading position. Since the heat is transferred to the lower mold in contact with the mold, heat radiation is improved. Therefore, a temperature difference occurs between the upper surface side and the lower surface side of the thin plate glass, and the amount of shrinkage of the glass differs between the upper surface side and the lower surface side, so that the thin plate glass warps. This warpage becomes particularly large when the press time is reduced for mass continuous production.

[0005] By the way, thin-walled glass is usually press-molded thicker than the final product, and then both surfaces are ground by lapping or the like to obtain a final product having a desired thickness and excellent flatness. At this time, if the thin plate glass is warped as described above, the thin plate glass bends when pressure is applied from both surface sides of the thin plate glass by the grinding plate. Therefore, even if the thin glass is ground in this state and the flat glass having a desired thickness and good flatness is obtained, when the pressure from the ground plate from both surfaces is released, the thin glass is warped again and becomes flat. A thin glass plate cannot be obtained.

[0006]

As described above, conventionally,
Warpage at the time of press molding remains even after double-side grinding as a final product, and there is a problem that a thin flat glass having good flatness cannot be obtained.

[0007]

According to the present invention, in order to solve the above-mentioned problems, a portion for receiving pressure from both surface sides is provided in a part of the thin glass plate.

[0008]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a thin glass sheet according to the present invention; FIG. 2 is a sectional view showing an embodiment of the thin plate glass of the present invention. As shown in this figure, a thin glass plate 11 according to an embodiment of the present invention
A thick portion 13 is provided at the outer peripheral end of the as a portion for receiving pressure from both surface sides. The thick portion 13 protrudes on both surface sides of the thin plate-shaped portion 12 (thin plate glass 11), and the protrusion amounts a and b are set to be larger than the expected warp amount f. As an example, the thick portion 13 is formed with the following numerical values. Referring to FIG. 2, the protrusion amount a = 0.1 mm,
Projection amount b = 0.15 mm, width c = 2.0 mm, thickness d =
1.7 mm, but the thickness e of the thin plate-shaped portion 12 is 1.4 m
m and an outer diameter of 66 mm.

Such a thin glass plate 11 is manufactured by press molding using an upper mold and a lower mold. FIG. 3 shows an upper mold 21 and a lower mold 31 for that purpose. The lower die 31 includes a cylindrical lower die main body 32 and a support rod 33 formed at the center of the lower surface of the lower die main body 32 to support the lower die main body 32. The upper surface is a press surface 34. However, the outer peripheral portion of the press surface 34 (the upper surface of the lower die body 32) is the thin plate glass 1 shown in FIG.
A concave portion 35 is formed to form one surface-side protrusion (lower mold-side protrusion) of one thick portion 13. The lower die 31 moves up and down by moving the support rod 33 up and down by driving means (not shown). The lower die 31 includes a trunk die 36 surrounding the lower die 31. The trunk die 36 projects inward from a cylindrical trunk body 37 and a lower end of the trunk die body 37. And an annular flange portion 38 formed as described above. The lower die 31 is provided slidably up and down on the inner peripheral surface of such a trunk die 36.

The upper mold 21 provided opposite to the lower mold 31
Is composed of a cylindrical upper mold body 22 and a support rod 23 formed at the center of the upper surface of the upper mold body 22 to support the upper mold body 22. The flat lower surface of the upper mold body 22 A pressing surface 24 is provided. Further, the support rod 2
The upper die 21 moves up and down by moving the 3 up and down by driving means (not shown). The upper die 21 includes a body die 25 so as to surround the upper die 21. The trunk mold 25 includes a cylindrical trunk body 26,
An annular flange 27 is formed at the upper end of the body 26 to protrude inward. The upper mold 21 is provided slidably up and down on the inner peripheral surface of the body mold 25. The outer diameter of the press surface 24 of the upper die 21 and the inner diameter of the body 26 are the same as those of the press surface 34 of the lower die 31.
From the outer diameter of the lower die 31 and the inner diameter of the lower die body 37.
Is formed to be smaller by the width of the concave portion 35 in the outer peripheral portion of the press surface 34. The upper mold 21 and the upper mold barrel 25 are formed of a heat-resistant material, such as graphite, a tungsten alloy, a nitride, a carbide, a heat-resistant metal, together with the lower mold 31 and the lower mold barrel 36. However, type 21,
When performing high-frequency heating, 25, 31, and 36 are limited to heat-resistant metals that can be heated by the high-frequency heating, and cast iron is particularly preferred because it has excellent strength and durability.

A method of press-molding the thin glass plate 11 of FIG. 2 using the upper mold 21 and the lower mold 31 configured as described above (an embodiment of the manufacturing method of the present invention) is shown in FIGS. This will be described with reference to FIG. In the press molding, first, as shown in FIG. 4 (a), a molten glass 42 of 1200 ° C. is poured from a platinum pipe 41 at a constant flow rate into a lower mold 31 (450 ° C.).
The molten glass 42 is supplied to the cutting blade 43 as shown in FIG.
Disconnect with The cut molten glass 42 becomes an ohashi-like glass gob that is rounded due to surface tension. Next, FIG.
As shown in (a), the upper mold body 25 is lowered, and the lower surface of the upper mold body 25 is brought into contact with the upper surface of the lower mold body 36. Next, as shown in FIG.
) Is lowered by sliding the inner peripheral surface of the barrel mold 25 and the glass gob is pressed by the upper mold 21 and the lower mold 31 for 1.0 second. As a result, the glass gob can be used as the lower mold body 36.
The thin glass 11 spreads over the entire space surrounded by. At this time, the upper mold 21 is lowered to a position where the upper mold 21 slightly enters the lower mold body mold 36. As a result, the thin plate-shaped portion 12 of the thin plate glass 11 is formed in the thin plate-shaped space between the upper mold 21 and the lower mold 31, and the space extending vertically outside the thin plate-shaped glass 11, that is, the lower mold 31 press surfaces 34
The lower end of the thin plate-shaped glass 11 spreads downward in the concave portion 35, and the upper die 21 enters the lower die body die 36 so as to expand upward inside the outer die part of the upper die 21. The thick part 13 is formed.

Next, as shown in FIG. 6A, the upper die 21 is raised by sliding on the inner peripheral surface of the trunk die 25. Further, FIG.
As shown in (b), the upper die 25 is raised. Thereafter, when the lower die 31 comes to the product take-out position by rotating a turntable (not shown) on which the lower die 31 is installed, the lower die 31 slides on the inner peripheral surface of the body die 36 to be lifted up, and integrally. The thin glass plate 11 is pushed up, and the thin glass plate 11 is removed from the lower mold 31 by a vacuum suction device (not shown).

The thin-plate glass 11 of FIG. 2 manufactured as described above is press-molded thicker than the final product, and then both surfaces are ground using grinding plates 51 and 52 as shown in FIG. By doing so, a final product having a desired thickness with excellent flatness is obtained. At this time, in a structure in which the thin plate glass is warped and the outer peripheral edge of the thin plate glass has no thick portion as in the conventional example, pressure is applied from both surface sides of the thin plate glass by the grinding plate. When this is obtained, the thin-plate glass bends, so even if the thin-plate glass is flattened by double-side grinding in that state, the thin-plate glass is warped again when the pressure from the grinding plates from both surface sides is released. However, a flat thin glass sheet cannot be obtained. On the other hand, according to the thin-plate glass 11 of the present invention shown in FIG. 2, the thick portion 13 is provided at the outer peripheral end portion, and the thick portion 13 as shown in FIG. Since the pressure from both surface sides by 52 is received, as shown in FIG.
Even when warpage occurs, the thin-plate glass 11 does not bend, and the double-sided grinding is performed in a state where the current shape is maintained. Glass can be obtained. When the warpage after double-sided grinding was actually measured, the warpage was 1-3 μm, which was greatly improved as compared with the conventional one (warpage was 5 to 10 μm). Further, according to the thin glass plate 11 of FIG. 2, since the thick portion 13 protrudes to both surface sides, even if it is warped in the opposite direction to that of FIG.
Can remove the influence of the pressure caused by the pressure, and can perform double-side grinding, so that the thin plate glass 11 having good flatness can be obtained. Furthermore, even when both surfaces are not parallel, the thin-plate glass 11 having excellent flatness can be finally obtained.

FIG. 7 is a sectional view showing another embodiment of the thin glass plate of the present invention. The thin plate glass 61 of the other embodiment has a thick portion 6 at the center of the thin plate portion 62.
3 is provided. The thick portion 63 is provided on the side that becomes the concave side when the expected warpage occurs and has a thickness equal to or larger than the expected amount of warpage. Thus, the thick portion 63 is replaced with the thin plate-like portion 62.
Also in the case of being provided at the center of the (thin-plate glass 61), as in the case of the thin-plate glass 11 in FIG. A thin-walled glass having good properties can be obtained. Since the upper surface (upper die side) is concave, the warp occurs upside down as shown in FIG. 7 due to heat radiation after press molding of the thin plate glass. The thick portion 63 can be formed by providing a recess at the center of the press surface of the upper die.

In the manufacturing method described with reference to FIG. 4 to FIG. 6, a plurality of lower dies are arranged, and the steps such as a glass gob supply step, a press molding step, and a molded article removal step are sequentially performed. For example, it is preferable to arrange individual lower molds on the circumference of the turntable and rotate the turntable so that the lower mold passes through each process, but it is designed to move in a linear direction. Is also good. Also,
The number of lower molds simultaneously provided in each step may be singular or plural. On the other hand, the upper mold is arranged to face the lower mold located in the step of press molding. Therefore,
The upper mold must have at least the same number as the lower mold used for one press molding, but may have more.

Next, the temperatures of the molding surfaces of the lower mold and the upper mold need to be adjusted to a predetermined temperature at the start of press molding. Here, the predetermined temperature of the mold refers to a temperature suitable for molding the glass material into a thin plate shape. This temperature is appropriately determined depending on the type of glass, the thickness, the size of the glass plate, and the like.

Further, in order to adjust the temperatures of the molding surfaces of the lower mold and the upper mold at the start of press molding to the predetermined temperature, means for heating the lower mold and the upper mold as necessary, and cooling. Measures are taken. As means for heating, for example, a method in which a plurality of nichrome heaters are arranged around a mold and heating is performed, a current is applied to a coil arranged so as to surround the periphery of the mold, and the mold made of a conductor is induction-heated. Although there are a method, a method of heating with a gas, and the like, a method by induction heating is preferable because uniform heating can be performed.
According to induction heating, unlike a method of heating one mold with a plurality of heat sources as in the case of heating with a nichrome heater, one or more molds can be heated with one coil, so the heat source temperature There is no problem of variation, and the mold can be uniformly heated by keeping the distance between the mold and the coil constant. Also, when using induction heating, even if induction heating is performed on both upper and lower molds,
Alternatively, it may be performed on either one of them, and when a trunk type is used, the present invention can be applied to the trunk type. here,
The current flowing through the coil during induction heating is preferably a high-frequency current. At low frequency currents, the device is large and noise can be a problem due to the human audible range.

On the other hand, the temperature of the molding die subjected to the press molding is higher than before the press molding due to the heat from the molten glass. Therefore, in order to press-mold any thin sheet glass under the same temperature conditions, it is necessary that the mold returns to the temperature before molding before being subjected to the next press molding. At this time, it is preferable to take some cooling means to return the temperature by taking some cooling means, except for naturally cooling to the time before being subjected to the next press molding and returning to the temperature before the press molding. . Therefore, a cooling unit is required at the same time as the heating unit. As the cooling means, a method of circulating water or air through the hollow portion of the mold, a method of spraying a liquid such as water onto the inner surface of the hollow portion of the mold to vaporize, and the like can be employed.
According to the method of spraying and vaporizing the liquid, the mold can be cooled by the heat of vaporization of the liquid, so that the cooling effect can be obtained with a smaller amount of liquid than in the method of circulating the liquid. Therefore, the method utilizing the heat of vaporization of water or the like is preferable not only from the viewpoint of the cooling effect, but also from the viewpoint of making the cooling device smaller. Furthermore, for example, when it takes time to cool the upper mold and it is not possible to cool the mold to a predetermined temperature after molding, a plurality of upper molds can be easily formed and any one of the upper molds can be press-molded. During the above, another upper mold may be cooled and a plurality of upper molds may be circulated.

In the method shown in FIGS. 4 to 6, the press is terminated when the glass is in a softened state. Therefore, at the end of press molding, the temperature of the thin glass plate is higher than the temperature of the molding die. Thus, the thin glass sheet and the mold are not in thermal equilibrium. However, since the mold is kept at a predetermined temperature in advance, the thin glass obtained by cooling after molding has a constant shape with a constant shape such as warpage, and is easily ground and polished. It has a shape.
In addition, since it is not necessary to cool the thin plate glass and the mold until a thermal equilibrium is reached, the molding time can be shortened. Further, for the purpose of shortening the press time, the press forming may be terminated at a temperature at which the center of the thin plate glass is equal to or higher than the softening point of the glass material. Further, since the thin glass sheet after the press molding is in a softened state, a step of correcting the warpage of the thin glass sheet may be performed after the press molding. The process of correcting the warpage of thin glass is
For example, by blowing air or the like only on one side of the thin glass sheet, the heat is unevenly removed, or by pressing again with a forming die having a forming surface similar to that of the upper die, the size of the warpage is increased. Is the process of correcting Also,
In the production method of the present invention, a molding method in which the surface temperatures of the upper and lower press surfaces are set near the glass transition point and the inner surface temperature of the body mold is set higher than the surface temperature of the press surface is applied. Is also possible.

Further, in order to form the molten glass into a thin plate, it is necessary to stretch the molten glass in the outer circumferential direction. Therefore, a solid lubricant is adhered to the molding surface of the molding die to lubricate the molten glass. It is preferable to increase the properties. At this time, the forming die for forming the thin-plate glass receives a higher amount of heat from the molten glass than in the case of forming a thick-walled glass by press forming, and thus has a high temperature. Therefore, the solid lubricant is preferably a heat-resistant one that does not lose lubricity even in a high temperature range. Such a heat-resistant solid lubricant is not particularly limited as long as it has excellent heat resistance, but boron nitride (BN) is preferable. Further, in order to obtain a sheet glass having excellent mechanical strength even in the case of a very thin thin sheet glass, a glass material having a high glass transition point may be used. In such a case, since the temperature of the mold also becomes considerably high, the heat resistance required of the solid lubricant becomes very high. Even in such a case, the BN powder is suitably used. By using a powdered heat-resistant solid lubricant, uniform adhesion of the glass to the molding surface and removal of excess can be easily performed.

The thin glass sheet obtained by the above-mentioned manufacturing method is subjected to mechanical processing such as grinding and polishing as outlined with reference to FIG. 1 to become a glass substrate for an information recording medium, for example. Hereinafter, the machining will be described in detail. As for the machining, specifically, the surface of the above glass is washed with water, and the following (1) roughing (rough polishing), (2) sanding (fine grinding, lapping), and (3) first polishing (Polished),
(4) Through each step of the second polishing (final polishing, polishing).

(1) Roughing Step First, both surfaces of the above glass substrate were ground one by one with a fine diamond grindstone. The load at this time is 10
It was about 0 kg. Thereby, the surface roughness of both surfaces of the glass substrate was finished to about 10 μm by Rmax (measured according to JIS B 0601). Next, a hole was made in the center of the glass substrate using a cylindrical grindstone, and the outer peripheral end face was also ground to reduce the diameter to 6 mm.
After 5 mmφ, a predetermined chamfering process was performed on the outer peripheral end surface and the inner peripheral surface.

(2) Sanding (Wrapping) Step Next, the glass substrate was sanded. This sanding step aims at improving dimensional accuracy and shape accuracy.
The sanding process was performed using a lapping device, and was performed twice while changing the grain size of the abrasive grains to # 400 and # 1000. More specifically, first, both surfaces of the glass substrate housed in the carrier were adjusted to a precision of 0 by using alumina abrasive grains having a grain size of # 400, setting the load to about 100 kg, and rotating the internal rotation gear and the external rotation gear. ~ 1μm, surface roughness (Rmax) 6μ
m. Next, lapping was performed by changing the particle size of the alumina abrasive grains to # 1000, and the surface roughness (R
max) was about 2 μm. The glass substrate that had been subjected to the sanding process was washed by sequentially immersing it in a neutral detergent and water washing tank.

(3) First polishing (polishing) step Next, a first polishing step was performed. This first polishing step is intended to remove scratches and distortion remaining in the above sanding step, and was performed using a polishing apparatus. For details, use a hard polisher (cerium pad M) as polisher (polishing powder).
HCl (manufactured by Speed Fam) under the following polishing conditions. Polishing liquid: cerium oxide + water Load: 300 g / cm ² (L = 238 kg) Polishing time: 15 minutes Removal amount: 30 μm Lower platen rotation speed: 40 rpm Upper platen rotation speed: 35 rpm Inner gear rotation speed: 14 rpm Outer gear rotation speed : 29 rpm The glass substrate after the above first polishing step was washed with a neutral detergent, pure water, pure water, IPA (isopropylene alcohol), IP
A (steam drying) was sequentially immersed in each cleaning tank for cleaning.

(4) Second Polishing Step Next, the polishing apparatus used in the first polishing step is performed, and the polisher is changed from a hard polisher to a soft polisher (Polyax: manufactured by Speed Fam). Carried out. The polishing conditions were the same as in the first polishing step, except that the load was 100 g / cm ² , the polishing time was 5 minutes, and the removal amount was 5 μm. The glass substrate after the second polishing step was immersed in each of a washing tank of a neutral detergent, a neutral detergent, pure water, pure water, IPA (isopropyl alcohol), and IPA (steam drying) to be washed. In addition, ultrasonic waves were applied to each cleaning tank. Thus, a disc-shaped glass substrate for an information recording medium having an outer diameter of 65 mmφ, a central hole diameter of 20 mmφ, a thickness of 0.5 mm, Rmax of 40 Å, and Ra of about 8 Å was obtained.

The glass substrate for an information recording medium manufactured by the above method constitutes a magnetic recording medium by sequentially laminating an underlayer, a magnetic layer, a protective layer and a lubricating layer on the glass substrate.

Here, as a material of the glass substrate of the magnetic recording medium, for example, aluminosilicate glass, soda lime glass, sodaaluminosilicate glass, aluminoborosilicate glass, borosilicate glass, quartz glass, chain silicate glass, or Glass ceramics such as crystallized glass are exemplified. further,
Preferably, a glass having the following composition is used. (1) Crystallized glass 1% by weight, SiO _2: 60 to 87%, Li ₂ O: 5%
To 20% Na ₂ O is 0 to 5% K ₂ O 0-10%
Na ₂ O and K ₂ O 0.5 to 10% in total, MgO is 0.5 to 7.5%, CaO is from 0 to 9.5%, SrO 0
-15%, BaO 0-13%, ZnO 0-13%,
B ₂ O ₃ is 0~10%, Al ₂ O ₃ is 0%, P ₂
O ₅ is 0.5 to 8%, TiO ₂ is 0 to 5%, ZrO ₂ is 0 to 3%, SnO ₂ is 0~3%, As ₂ O ₃ and Sb ₂ O
₃ contains 0 to 2% in total, and contains 0 to 5% of a fluoride of one or more metal elements of the metal oxide as a total amount of F, and optionally contains V ₂ O ₅ , CuO, and Mn as coloring components.
O ₂ , Cr ₂ O ₃ , CoO, MoO ₃ , NiO, Fe ₂
O ₃ , TeO ₂ , CeO ₂ , Pr ₂ O ₃ , Nd ₂ O ₃ ,
At least one selected from the group of Er ₂ O ₃ 0~5%
A crystallized glass containing, as a main crystal, lithium disilicate, optionally α-cristobalite, α-quartz, lithium monosilicate, β-spodumene, etc., and having a crystal grain size of 3.0 μm or less. (2) crystallized glass 2 wt% display, SiO ₂ is 45 to 75%, CaO is 4
30% Na ₂ O is 2 to 15% K ₂ O 0 to 20%
Al ₂ O ₃ 0-7%, MgO 0-2%, ZnO 0
２2%, SnO ₂ is 0 to 2%, Sb ₂ O ₃ is 0 to 1%,
B ₂ O ₃ is 0~6%, ZrO ₂ is 0~12%, Li ₂ O
Contains 0 to 3%, a fluoride of one or more metal elements of the metal oxide described above in a total amount of 3 to 12% of F, and optionally contains Cr ₂ O ₃ , Co ₃ O _{4 and the} like as coloring components. And
A crystallized glass containing canasite or potassium fluororichterite as a main crystal and having a crystal grain size of 1.0 μm or less. (3) a glass 3 wt% display, SiO ₂ is 62~75%, Al ₂ O ₃ is 4 to 18%, ZrO ₂ is 0 to 15%, Li ₂ O is 3-1
Glass containing 2% and ₃ to 13% of Na2O.

The surface of such a glass substrate can be subjected to a chemical strengthening treatment by a low-temperature ion exchange method for the purpose of improving impact resistance, vibration resistance and the like. Here, the chemical strengthening method is not particularly limited as long as it is a conventionally known chemical strengthening method. For example, low-temperature chemical strengthening in which ion exchange is performed in a region not exceeding a transition temperature from the viewpoint of a glass transition point is preferable. . Examples of the alkali molten salt used for chemical strengthening include potassium nitrate and sodium nitrate, and nitrates obtained by mixing them.

As the underlayer, for example, Cr, Mo, T
a, an underlayer made of at least one material selected from nonmagnetic metals such as Ti, W, V, B, and Al. In the case of a magnetic layer containing Co as a main component, it is preferable to use Cr alone or a Cr alloy from the viewpoint of improving magnetic properties. The underlayer is not limited to a single layer, and may have a multilayer structure in which the same or different layers are stacked. For example, Cr / Cr, Cr / CrMo, Cr / CrV, Cr
V / CrV, Al / Cr / CrMo, Al / Cr / C
r, a multilayer base layer of Al / Cr / CrV, Al / CrV / CrV and the like.

As the magnetic layer, for example, CoPt, CoCr, CoNi, CoNiCr, C
oCrTa, CoPtCr, CoNiPt, CoNi
CrPt, CoNiCrTa, CoCrTaPt, Co
A magnetic thin film such as CrPtSiO may be used. The magnetic layer is made of a non-magnetic film (for example, Cr, CrMo, Cr).
V, etc. to reduce noise (for example, CoPtCr / CrMo / CoPtCr, CoC
rTaPt / CrMo / CoCrTaPt). As a magnetic layer corresponding to a magnetoresistive head (MR head) or a large magnetoresistive head (GMR head),
Co, Y, Si, rare earth elements, Hr, Ge, S
An impurity element selected from n and Zn or an element containing an oxide of these impurity elements is also included. As the magnetic layer, in addition to the above, ferritic, iron - rare-earth and, in a non-magnetic film made of SiO _2, BN F
Granules having a structure in which magnetic particles such as e, Co, FeCo, and CoNiPt are dispersed may be used. Also,
The magnetic layer may be of any of an internal recording type and a vertical recording type.

Examples of the protective layer include a Cr film, a Cr alloy film, a carbon film, a zirconia film, and a silica film. These protective layers can be continuously formed with an underlayer, a magnetic layer, and the like by an in-line type sputtering apparatus. Also,
These protective layers may be a single layer, or may have a multilayer structure composed of the same or different films. further,
Another protective layer may be formed on the protective layer or in place of the protective layer. For example, instead of the above protective layer,
Colloidal silica fine particles may be dispersed and applied in a state where tetraalkoxylan is diluted with an alcohol-based solvent on the Cr film, followed by firing to form a silicon oxide (SiO ₂ ) film.

The lubricating layer is prepared, for example, by diluting perfluoropolyether (PFPE), which is a liquid lubricant, with a solvent such as Freon, and applying it to the medium surface by dipping, spin coating, or spraying. It is formed by performing heat treatment.

The present invention will be described in detail above. The thin glass plates 11 and 61 shown in FIGS. 1 and 7 are disk-shaped, that is, circular, but may be formed in various shapes such as a square.
Further, the structure of the molding die for press-molding a thin glass plate having a thick portion is not limited to the structure shown in FIG.

[0034]

As described above, according to the thin plate glass of the present invention and the method of manufacturing the same, a portion for receiving pressure from both surface sides is provided in a part of the thin plate glass. After the double-side grinding as a product, the warpage is almost removed, and a thin plate glass having good flatness can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a state where a thin plate glass according to an embodiment of the present invention is ground on both sides with a grinding plate.

FIG. 2 is a sectional view showing an embodiment of the thin plate glass of the present invention.

FIG. 3 is a sectional view showing an upper mold and a lower mold for producing the thin plate glass of FIG. 2;

FIG. 4 is a cross-sectional view showing a manufacturing process of the thin plate glass of FIG. 2 as an embodiment of the method of manufacturing the thin plate glass of the present invention.

FIG. 5 is a sectional view showing the manufacturing process.

FIG. 6 is a sectional view showing the same manufacturing process.

FIG. 7 is a cross-sectional view showing another embodiment of the thin plate glass of the present invention.

[Description of Signs] 11, 61 Thin glass plate 13, 63 Thick part 21 Upper die 31 Lower die

Claims (5)

[Claims] 1. A thin plate glass characterized in that a portion for receiving pressure from both surface sides is provided in a part. 2. The thin glass sheet according to claim 1, wherein the pressure receiving portion is formed of a thick portion having a greater thickness than other portions. 3. A method for producing a thin sheet glass, comprising: producing the thin sheet glass according to claim 1 by press molding. 4. A method for producing a glass substrate for an information recording medium, wherein at least the thin plate glass according to claim 1 or the thin plate glass produced by the method according to claim 3 is polished. . 5. A magnetic recording medium comprising at least a magnetic layer formed on a glass substrate for an information recording medium manufactured by the manufacturing method according to claim 4.