JP2012101990A - Method of manufacturing glass blank for glass substrate of magnetic recording medium, method of manufacturing glass substrate of magnetic recording medium, method of manufacturing magnetic recording medium, and apparatus for manufacturing glass blank for glass substrate of magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a glass blank having excellent evenness and plate thickness deviation.SOLUTION: The present invention is a method of manufacturing a glass blank for a glass substrate of a magnetic recording medium in which a press-molding process in which a pair of press-molding molds 50 and 60 press-mold a piece of molten glass mass that is dropped using the pair of press-molding molds 50 and 60 that include press-molding mold main bodies 52 and 62 and guide members 54 and 64 includes: a first step of pressing the piece of molten glass mass until the guide members 54 and 64 of the pair of press-molding molds 50 and 60 disposed opposing each other in a horizontal direction come into contact with each other; and a second step of further continuously pressing a plate-shaped glass 26 by the press-molding mold main bodies 52 and 62 in a state in which the guide members 54 and 64 are in contact with each other. The present invention also relates to a method of manufacturing a glass substrate of a magnetic recording medium and a method of manufacturing a magnetic recording medium using the method of manufacturing a glass blank for a glass substrate of a magnetic recording medium.

Description

  The present invention relates to a method for producing a glass blank for a magnetic recording medium glass substrate, a method for producing a magnetic recording medium glass substrate, a method for producing a magnetic recording medium, and an apparatus for producing a glass blank for a magnetic recording medium glass substrate.

  As a method for producing a magnetic recording medium glass substrate (magnetic disk glass substrate), typically, (1) a method of producing a molten glass lump through a press forming step of press forming a molten glass lump with a pair of press forming dies (hereinafter referred to as a method) , And may be referred to as a “press method.” (See Patent Documents 1 and 2), and (2) A method of producing a sheet glass obtained by a float method, a downdraw method, or the like through a step of cutting into a disk shape. (Hereinafter, it may be referred to as “sheet-like glass cutting method”. See Patent Document 3).

  In the conventional sheet-shaped glass cutting method exemplified in Patent Document 3, a lapping process (rough polishing process) and a polishing process (precision) are performed as a main surface polishing process after a disk processing process for processing the sheet glass into a disk shape. Polishing process) to obtain a magnetic recording medium glass substrate. However, the sheet-like glass cutting method disclosed in Patent Document 3 discloses that the lapping process (rough polishing process) is omitted as the main surface polishing process and only the polishing process (precision polishing process) is performed.

  On the other hand, in the conventional press systems exemplified in Patent Documents 1 and 2, etc., the molten glass lump is usually arranged on the lower mold, and then vertically with respect to the molten glass lump by the upper mold and the lower mold. After a press molding process is performed using a method of pressing a molten glass lump by applying a pressing force from the direction (hereinafter sometimes referred to as “vertical direct press”), the magnet is further subjected to a lapping process, a polishing process, etc. A recording medium glass substrate is obtained.

  Here, in the press method shown in Patent Document 1, the lapping process is omitted by using a high-rigidity material as the material of the upper mold, the lower mold, and the parallel spacer disposed between the upper mold and the lower mold. It has also been proposed.

  In the press method shown in Patent Document 2, the press molding process is a method in which a pressing force is applied from the horizontal direction to the molten glass lump (hereinafter referred to as “horizontal”) by a pair of press molds arranged to face each other in the horizontal direction. It may be referred to as “direct press”).

JP 2003-54965 A (claims, paragraph numbers 0040 and 0043, FIGS. 4 to 8, etc.) Patent No. 4380379 (paragraph 0031, FIGS. 1 to 9 etc.) Japanese Unexamined Patent Publication No. 2003-36528 (FIGS. 3 to 6, FIG. 8, etc.)

  On the other hand, in order to increase the productivity of the magnetic recording medium glass substrate, the lapping process performed mainly for ensuring the flatness and thickness uniformity of the magnetic recording medium glass substrate and adjusting the thickness is omitted. Or shortening the time is very effective. This is because, in the lapping process, a lapping apparatus is required for the implementation, and the man-hour for producing the magnetic recording medium glass substrate is increased and the processing time is increased. In addition, the lapping process may cause cracks on the glass surface, and the present situation is that the omission of the lapping process is being studied. Here, in terms of omitting the wrapping process or shortening the time, the sheet-like glass cutting method is compared with the press method, and the sheet-like glass with high flatness produced by the float method, down-draw method, etc. is used. Therefore, the sheet-like glass cutting method in which processing is performed is more advantageous. However, the press method has an advantage that the glass utilization efficiency is higher than the sheet-like glass cutting method.

  When manufacturing a magnetic recording medium by post-processing a glass blank manufactured using a vertical direct press, in order to omit or shorten the lapping process, the thickness deviation of the glass blank is reduced. At the same time, it is necessary to increase the flatness. Here, when manufacturing a glass blank by the vertical direct press, the temperature of the lower mold is set to a temperature sufficiently lower than the temperature of the molten glass lump so that the high-temperature molten glass lump does not melt. Therefore, the molten glass block is deprived of heat from the surface in contact with the lower mold until the press molding is started after the molten glass block is arranged on the lower mold. Therefore, the viscosity of the lower surface of the molten glass block arranged on the lower mold is locally increased. As a result, the press molding is performed on the molten glass lump in which a large viscosity distribution (temperature distribution) is generated, and thus a portion that is difficult to stretch is generated by pressing. Moreover, the cooling rate after press molding also differs for each part of the glass molded body that has been press-molded and stretched into a plate shape. For this reason, in the glass blank produced using a vertical direct press, a plate | board thickness deviation increases or flatness tends to fall. Considering the mechanism described above, as shown in Patent Document 1, even if it is a vertical direct press using a parallel spacer, drastic increase in plate thickness deviation and flatness of glass blank are drastically reduced. It is difficult to suppress.

  On the other hand, in the horizontal direct press exemplified in Patent Document 2, when the molten glass lump comes into contact with the press mold, it is pressed into a plate shape substantially simultaneously. That is, compared with the vertical direct press, the horizontal direct press has a uniform viscosity distribution of the molten glass lump at the time of press molding, and therefore it is easy to stretch the molten glass lump thinly and uniformly. For this reason, in comparison with the vertical direct press, in principle, it is considered that the horizontal direct press is easier to drastically suppress the increase in the thickness deviation and the decrease in flatness of the glass blank.

  However, as the recording density of magnetic recording media has been further improved in recent years, the thickness deviation and flatness of glass magnetic recording media glass substrates and glass blanks used for the production of magnetic recording media should be further improved. Is required. On the other hand, in the integrated press mold used in the horizontal direct press exemplified in Patent Document 2, gravity acts in a direction parallel to the press molding surface. For this reason, compared to a vertical direct press in which the lower die is substantially fixed and the upper die moves in a direction parallel to the direction of gravity, the horizontal direct press has a press molding surface relative to the movement direction of the press die. Tend to tilt. Therefore, in the horizontal direct press, in order to reduce the plate thickness deviation, it is necessary to perform press forming in a state where the press forming surfaces facing each other are maintained in parallel. For this purpose, the drive of the press mold in the horizontal direction during press molding must be controlled very precisely. However, even if the press mold drive device is improved, there is a limit to drive the press mold more precisely, so it is difficult to improve the thickness deviation, and the cost becomes high. Lack. For this reason, even if the press mold illustrated in Patent Document 2 is used, it is difficult to further improve the thickness deviation and the flatness.

  The present invention has been made in view of the above circumstances, and a glass blank for a magnetic recording medium glass substrate capable of producing a glass blank having a smaller thickness deviation and flatness even when a glass blank is produced by horizontal direct pressing. Manufacturing method, and magnetic recording medium glass substrate manufacturing method, magnetic recording medium manufacturing method, and apparatus for manufacturing glass blank for magnetic recording medium glass substrate using the manufacturing method of glass blank for magnetic recording medium glass substrate The task is to do.

The above-mentioned subject is achieved by the following present invention. That is,
The method for producing a glass blank for a magnetic recording medium glass substrate according to the present invention includes a first press mold and a first press mold in which a molten glass lump that is falling is arranged opposite to the direction intersecting the falling direction of the molten glass lump. At least a press molding step of press molding with a second press mold is used to manufacture a glass blank for a magnetic recording medium glass substrate, and at least a first press mold has a press mold body having a press molding surface, and a press At the time of molding, the first press mold and the second press mold are brought into contact with a part of the press mold disposed opposite to the press molding surface when being extruded to the side of the press mold disposed opposite to the press molding surface. A guide member having at least a function of maintaining a substantially constant distance between the press molding surfaces of the two press molds. The molten glass lump is formed into sheet glass by bringing the first press mold and the second press mold close to each other until the guide member of the mold and the second press mold come into contact with each other. In a state where the first step, the guide member of the first press mold, and the second press mold are in contact with each other, the press mold body of the first press mold, the second press mold, And a second step of continuing to press the sheet glass further.

  One embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention is that a first press mold and a second press mold each have a press mold body having a press molding surface, and press molding. Sometimes the first press mold and the second press mold are brought into contact with a part of the press mold disposed opposite to the press molding surface when being pushed out to the press mold side disposed opposite to the press molding surface. At least a guide member having a function of keeping the distance between the press molding surfaces of the press mold substantially constant, wherein the first step includes the guide member of the first press mold and the second press molding. The first press mold and the second press mold are brought close to each other until the guide member of the mold comes into contact, and the second step is a guide portion of the first press mold In a state where the guide member of the second press mold is in contact with the press mold body of the first press mold and the press mold body of the second press mold, Furthermore, it is preferable to carry out by continuing pressing.

  In another embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention, the molten glass is suspended from the glass outlet, and the tip of the molten glass flow that continuously flows out downward in the vertical direction is provided. It is preferable to include a molten glass lump forming step of forming a molten glass lump by cutting.

  In another embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention, the viscosity of the molten glass is preferably in the range of 500 dPa · s to 1050 dPa · s.

  In another embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention, the first press mold and the second press mold are opposed to the direction perpendicular to the falling direction of the molten glass lump. It is preferable that they are arranged.

  In another embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention, the duration of the second step is controlled so that the flatness of the glass blank for the magnetic recording medium glass substrate is 10 μm or less. Is preferred.

  In another embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention, the duration of the second step is set so that the temperature of the plate glass at the end of the second step is at least the plate glass. It is preferable to select the temperature to be equal to or lower than the temperature obtained by adding 10 ° C. to the strain point of the glass material constituting the glass.

  In another embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention, the temperature of the press molding surface of the first press mold immediately before the first step and the second press molding It is preferable that the absolute value of the difference between the temperature of the press-molded surface of the mold is in the range of 0 ° C to 10 ° C.

  In another embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention, the surface of the press molding surface of the first press mold and the second press mold immediately before the first step is performed. The absolute value of the internal temperature difference is preferably in the range of 0 ° C to 100 ° C.

  In another embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention, at least during the press molding step, the temperature of the press molding surface of the first press mold and the second press molding The temperature of the press molding surface of the mold is substantially the same, and the press molding surface of the first press molding die and the press molding surface of the second press molding die are It is preferable to press-mold the molten glass lump after contact at substantially the same time.

  In another embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention, the temperature of the sheet glass is at least the temperature obtained by adding 10 ° C. to the strain point of the glass material constituting the sheet glass. The second step described above is preferably continued.

  In another embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention, it is preferable that the pressing pressure in the second step is decreased with time.

  In another embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention, the press pressure is set to the temperature of the sheet glass sandwiched between the first press mold and the second press mold. However, it is preferable to decrease when the glass material constituting the plate-like glass is lowered to the range of the yield point ± 30 ° C.

  In another embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention, the flatness of the glass blank for a magnetic recording medium glass substrate is preferably 10 μm or less.

  In another embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention, the flatness of the glass blank for a magnetic recording medium glass substrate is preferably 4 μm or less.

  In another embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention, at least a region in contact with the glass sheet on the press molding surface of the first press mold and the second press mold is approximately. A flat surface is preferred.

  In another embodiment of the method for producing a glass blank for a magnetic recording medium glass substrate of the present invention, each of the first press mold and the second press mold includes a press mold body and a guide member. A first extrusion member that is simultaneously extruded to a press mold side that is orthogonal to the surface and is disposed opposite to the press molding surface, and that faces the guide member and the press molding surface by the first extrusion member. After the part of the placed press mold comes into contact, the press mold body is pushed in the direction perpendicular to the press molding surface and the press mold side arranged opposite to the press molding surface. It is preferable to further include an extrusion member.

  The method of manufacturing a magnetic recording medium glass substrate of the present invention includes a first press mold and a second press in which a molten glass lump that is falling is arranged opposite to the direction intersecting the falling direction of the molten glass lump. After producing a glass blank for a magnetic recording medium glass substrate through at least a press molding step of press molding with a molding die, the magnetic recording medium glass substrate is subjected to at least a polishing step for polishing the main surface of the glass blank for magnetic recording medium. When at least the first press mold is manufactured and pressed into the press mold surface having the press mold surface and the press mold side disposed opposite to the press mold surface during press molding, The distance between the press forming surfaces of the first press forming die and the second press forming die is substantially constant by contacting a part of the press forming die arranged oppositely. A guide member having at least a function of maintaining the first press mold and the second press mold until the guide member of the first press mold contacts the second press mold. The first step of forming the molten glass lump into sheet glass by bringing the two press molds close to each other, the guide member of the first press mold, and the second press mold in contact with each other And a second step of further pressing the sheet glass with the press mold body of the first press mold and the second press mold.

  In one embodiment of the method for producing a magnetic recording medium glass substrate of the present invention, the flatness of the glass blank for a magnetic recording medium glass substrate and the flatness of the magnetic recording medium glass substrate are preferably substantially the same.

  The method for producing a magnetic recording medium of the present invention includes a first press mold and a second press mold in which a molten glass lump that is falling is arranged opposite to the direction intersecting the falling direction of the molten glass lump. After producing a glass blank for a magnetic recording medium glass substrate through at least a press molding step for press molding, a magnetic recording medium glass substrate is produced through at least a polishing step for polishing the main surface of the glass blank for magnetic recording medium. Furthermore, a magnetic recording medium is manufactured through at least a magnetic recording layer forming step of forming a magnetic recording layer on the magnetic recording medium glass substrate, and at least the first press mold has a press mold surface having a press molding surface. And a press disposed opposite to the press molding surface when being pressed to the press mold side disposed opposite to the press molding surface during press molding. And at least a guide member having a function of maintaining a substantially constant distance between the press molding surfaces of the first press mold and the second press mold by contacting a part of the mold. The process moves the molten glass lump by bringing the first press mold and the second press mold close to each other until the guide member of the first press mold and the second press mold come into contact with each other. In a state where the first step of forming into a sheet glass, the guide member of the first press mold, and the second press mold are in contact, the press mold body of the first press mold, And a second step of continuing to press the sheet glass by a second press mold.

  In one embodiment of the method for producing a magnetic recording medium of the present invention, it is preferable that the flatness of the glass blank for a magnetic recording medium glass substrate and the flatness of the magnetic recording medium glass substrate are substantially the same.

  An apparatus for manufacturing a glass blank for a magnetic recording medium glass substrate according to the present invention includes a molten glass outflow pipe having an outlet for dropping a molten glass flow downward in the vertical direction, and a molten glass flow out of the molten glass outflow pipe. It is a direction substantially perpendicular to the direction of drooping and is disposed opposite to both sides of the direction of dripping of the molten glass flow. A pair of shear blades that form a lump, and a direction that is substantially orthogonal to the direction in which the molten glass lump falling downward in the vertical direction is disposed opposite to both sides of the direction in which the molten glass lump is dropped, At least a first press mold and a second press mold that press-mold the molten glass lump into sheet glass by sandwiching the molten glass lump from both sides, and at least The first press mold is placed opposite to the press mold surface when the press mold body having a press mold surface and the press mold side disposed opposite to the press mold surface are pressed during press molding. A guide member having at least a function of maintaining a substantially constant distance between the press molding surfaces of the first press mold and the second press mold by contacting a part of the press mold; And a guide member in a direction orthogonal to the press molding surface, and a first extrusion member that simultaneously extrudes to the press molding die side disposed opposite to the press molding surface, and the first extrusion member, After the guide member and a part of the press mold placed opposite to the press molding surface come into contact, the press mold body is disposed in a direction perpendicular to the press molding surface and opposed to the press molding surface. Characterized in that it comprises a second pushing member for pushing the pressing mold side, at least.

  According to the present invention, when manufacturing a glass blank by horizontal direct press, a method for manufacturing a glass blank for a magnetic recording medium glass substrate capable of manufacturing a glass blank having a smaller plate thickness deviation and flatness, and the magnetic recording A magnetic recording medium glass substrate manufacturing method, a magnetic recording medium manufacturing method, and a glass blank manufacturing apparatus for a magnetic recording medium glass substrate using the method for manufacturing a glass blank for a medium glass substrate can be provided.

In an example of the manufacturing method of the glass blank of this embodiment, it is a schematic cross section explaining a part of all the processes. In an example of the manufacturing method of the glass blank of this embodiment, it is a schematic cross section explaining the other part of all the processes. It is a schematic cross section which shows an example of the molten glass lump in the fall. In an example of the manufacturing method of the glass blank of this embodiment, it is a schematic cross section explaining the other part of all the processes. In an example of the manufacturing method of the glass blank of this embodiment, it is a schematic cross section explaining the other part of all the processes. In an example of the manufacturing method of the glass blank of this embodiment, it is a schematic cross section explaining the other part of all the processes. In an example of the manufacturing method of the glass blank of this embodiment, it is a schematic cross section explaining the other part of all the processes. In an example of the manufacturing method of the glass blank of this embodiment, it is a schematic cross section explaining the other part of all the processes. In an example of the manufacturing method of the glass blank of this embodiment, it is a schematic cross section explaining the other part of all the processes. It is a schematic cross section which shows an example of the more concrete structure of the press mold used for the manufacturing method of the glass blank of this embodiment. It is a schematic cross section which shows the other example of the press-molding die used for the manufacturing method of the glass blank of this embodiment. It is a schematic cross section of the press mold used in Comparative Example A1. It is a schematic cross section of the press mold used in Comparative Example A2.

(Method of manufacturing glass blank for magnetic recording medium glass substrate and manufacturing apparatus using the same)
The method for producing a glass blank for a magnetic recording medium glass substrate of the present embodiment (hereinafter sometimes abbreviated as “a method for producing a glass blank”) causes the molten glass lump that is falling to fall in the direction in which the molten glass lump falls. At least through a press molding step of press molding with a first press mold and a second press mold disposed opposite to each other in a direction intersecting the glass blank for a magnetic recording medium glass substrate (hereinafter, “glass blank”) Is manufactured in some cases.

  Here, at least when the first press mold is extruded to the press mold body having the press mold surface and the press mold side disposed opposite to the press mold surface during press molding, A guide member having at least a function of maintaining a substantially constant distance between the press molding surfaces of the first press mold and the second press mold by contacting a part of the press molds arranged opposite to each other. At least.

  Then, the press molding step brings the first press mold and the second press mold close to each other until the guide member of the first press mold and the second press mold come into contact with each other. Press molding of the first press mold with the first step of molding the molten glass lump into a sheet glass, the guide member of the first press mold, and the second press mold And a second step of further pressing the sheet glass with the mold body and the second press mold.

  Here, in the first step of the press molding process, the first press mold and the second press mold are brought close to each other. For this reason, a molten glass lump is press-molded by the first press-molding die and the second press-molding die to form a plate glass. Further, by bringing the first press mold and the second press mold close to each other, the guide member of the first press mold and the second press mold are brought into contact with each other. Therefore, at this time, the distance between the press molding surfaces of the first press mold and the second press mold is kept substantially constant. For this reason, the plate | board thickness deviation of the plate glass of the state clamped between press molding surfaces can be made very small. Therefore, as a result, the thickness deviation of the obtained glass blank can be made very small.

  However, in the state immediately after the first step is finished, the glass sheet is in a state where it is highly fluid at high temperature and easily deformed. For this reason, immediately after the first step is finished, when the first press mold and the second press mold are separated from each other and the sheet glass is taken out, both surfaces of the sheet glass are formed by any member. Is not supported, the plate glass is easily deformed. For this reason, the flatness of the produced glass blank deteriorates. Therefore, even after the first step is finished, for a while, the state in which the guide member of the first press mold and the second press mold are kept in contact with each other and the sheet glass is maintained. It is thought that it is preferable to support both sides by a press molding surface. Thereby, the flatness of the glass blank can be improved by cooling the plate glass while preventing both of the plate glass from being deformed in a state where both surfaces of the plate glass are supported by the press-molded surface, and the fluidity is lowered or lost. This is because it is considered that deterioration can be suppressed.

  However, the glass sheet in contact with the press molding surface shrinks in the process of being deprived of heat by the press mold and being cooled. For this reason, if the distance between the press molding surfaces immediately after the completion of the first step continues to be maintained thereafter, a gap is formed between the press molding surface and the sheet glass. Therefore, considering the shrinkage of the sheet glass, it is difficult to always support both surfaces of the sheet glass with the press-molded surface. Therefore, even if the state where the guide member of the first press mold and the second press mold are kept in contact with each other for a while after the first step is finished, It is difficult to suppress deformation.

  However, in the second step, with the guide member of the first press mold and the second press mold in contact, the press mold body of the first press mold and the second press Continue pressing the glass sheet with the mold. That is, the sheet glass is further pressed so that only the press mold body of the first press mold is brought closer to the press molding surface side of the second press mold. For this reason, even if the plate-like glass contracts in the thickness direction, the press-molding surface keeps in close contact with both sides of the plate-like glass without a gap, and supports both sides of the plate-like glass. Therefore, if the sheet glass is cooled in such a state that the both sides of the sheet glass are always supported by the press-molded surface and taken out after the fluidity is reduced or lost, the flatness of the glass blank is further deteriorated. It can be reliably suppressed.

  In the second step, the press molding surface of the press mold composed of a solid member having higher thermal conductivity than the gas such as air existing in the gap and the plate glass are kept in close contact with each other without any gaps. Glass heat is efficiently taken away by the press mold. For this reason, the fluidity | liquidity fall of the plate glass during press molding is accelerated | stimulated more compared with the case where a clearance gap is formed between a press molding surface and plate glass. Therefore, at the time when the glass sheet and the press-molded surface are separated (the time when the second step is completed), the flowability of the glass sheet is further lowered and becomes in a state where it is difficult to deform or cannot be deformed. . Therefore, also from this viewpoint, the deformation of the sheet glass after press molding hardly occurs, and the flatness of the glass blank can be further reduced.

  In the glass blank manufacturing method of the present embodiment, only the first press mold may include at least a press mold body and a guide member. In this case, as the second press mold, for example, a press mold having a columnar body with one end surface constituting the press mold surface can be used. In this case, in the first step, the guide member of the first press mold contacts the press molding surface of the second press mold. And in the second step, in a state where the guide member of the first press mold and the press molding surface of the second press mold are in contact, the press mold body of the first press mold, The sheet glass is further pressed by the second press mold (hereinafter, such a press molding process may be referred to as a “first press process”).

  Further, in the glass blank manufacturing method of the present embodiment, each of the first press mold and the second press mold faces the press mold body having the press mold surface and the press mold surface during press molding. Pressing of the first press mold and the second press mold by contacting with a part of the press mold placed opposite to the press molding surface when extruded to the arranged press mold side And at least a guide member having a function of keeping the distance between the surfaces substantially constant. In this case, the first step is to move the first press mold and the second press mold until the first press mold guide member and the second press mold guide member contact each other. This is done by bringing them closer together. And the second step is a state where the first press mold die guide member and the second press mold guide member are in contact with each other. It is carried out by further pressing the sheet glass with the press mold body of the second press mold (hereinafter, such a press molding process may be referred to as a “second press process”). .

  In carrying out the second step, a press mold including at least a press mold main body and a guide member used in both the first press process and the second press process is a press mold main body. It is particularly preferable that the guide member has a function of being able to move separately to the other press-molding die disposed opposite to the guide member.

  The manufacturing method of the glass blank of this embodiment can be implemented by any process of a 1st press process and a 2nd press process. However, in order to obtain a glass blank having the same thickness deviation and flatness, the time required for press molding can be further shortened compared to the first press process, for example, about 1/3. It is particularly preferable that the second pressing process is performed from the viewpoint of enabling this. This is because the structure of the pair of press molds used in the second press process is more similar to or the same structure in the second press process than in the first press process. This is because the cooling of the sheet glass to be performed can be performed more symmetrically from both sides.

  Although the manufacturing method of the glass blank of this embodiment demonstrated above will not be specifically limited if it includes at least a press molding process, Usually, it is preferable to have a molten glass lump formation process. Moreover, after passing through the press molding process, a take-out process of taking out the sheet glass by separating the first press mold and the second press mold is performed. Below, each process is demonstrated in detail including a molten-glass lump formation process and an extraction process. In the following description, description of points already described above is omitted.

-Molten glass lump formation process-
In the molten glass lump forming step, a molten glass lump that is an object of press molding is produced. Although it is not particularly limited as a method for producing a molten glass lump, usually, by dropping the molten glass from the glass outlet and cutting the tip of the molten glass flow that continuously flows out downward in the vertical direction, A molten glass lump is formed. A pair of shear blades can be used for cutting the tip of the molten glass flow. The viscosity of the molten glass is not particularly limited as long as the viscosity is suitable for cutting of the tip portion or press molding, but is usually controlled to a constant value within a range of 500 dPa · s to 1050 dPa · s. It is preferable. In addition, it is preferable that the viscosity of the molten glass lump immediately before press molding is also within the above range.

  Next, a specific example of the molten glass lump forming step will be described in detail with reference to the drawings. In the molten glass lump forming step, as shown in FIG. 1, a molten glass flow 20 is provided from a glass outlet 12 provided at the lower end of a molten glass outlet pipe 10 whose upper end is connected to a molten glass supply source (not shown). Is continuously discharged downward in the vertical direction. On the other hand, below the glass outlet 12, the first shear blade (lower blade) 30 and the second shear blade (upper blade) 40 are melted on both sides of the molten glass flow 20, respectively. It arrange | positions in the direction substantially orthogonal to the central axis D of the direction where the glass flow 20 hangs down. The lower blade 30 and the upper blade 40 are each orthogonal to the central axis D, and in the drawing, are orthogonal to the direction of the arrow X1 that proceeds from the left side to the right side, and the central axis D. And it moves to the front-end | tip part 22 side of the molten glass flow 20 from the both sides of the molten glass flow 20 by moving to the arrow X2 direction which advances from the right side to the left side in the figure. In addition, the viscosity of the molten glass flow 20 is controlled by adjusting the temperature of the molten glass outflow pipe 10 or the molten glass supply source upstream thereof.

  Further, the lower blade 30 and the upper blade 40 are provided on the substantially plate-like main body portions 32 and 42 and the end portions of the main body portions 32 and 42, and a molten glass flow that continuously flows downward in the vertical direction. It has the blade parts 34 and 44 which cut | disconnect the front-end | tip part 22 of 20 from the direction substantially orthogonal to the direction where the molten glass flow 20 hangs down. The upper surface 34U of the blade portion 34 and the lower surface 44B of the blade portion 44 form a surface that substantially coincides with the horizontal plane, and the lower surface 34B of the blade portion 34 and the upper surface 44U of the blade portion 44 intersect with the horizontal plane. An inclined surface is formed. In addition, the lower blade 30 and the upper blade 40 are arranged so that the upper surface 34U of the blade part 34 and the lower surface 44B of the blade part 44 are at substantially the same height with respect to the vertical direction.

  Next, as shown in FIG. 2, the upper blade 34U and the lower surface 44B of the blade portion 44 are moved further by moving the lower blade 30 and the upper blade 40 in the directions of the arrow X1 and the arrow X2, respectively. Move the lower blade 30 and the upper blade 40 in the horizontal direction so that they partially overlap with each other with almost no gap. That is, the lower blade 30 and the upper blade 40 are perpendicularly intersected with the central axis D. As a result, the lower blade 30 and the upper blade 40 penetrate into the molten glass flow 20 to the vicinity of the central axis D, and the tip 22 is cut as a substantially spherical molten glass lump 24. FIG. 2 shows a state in which the front end portion 22 is separated from the main body portion of the molten glass flow 20 as a molten glass lump 24. And as shown in FIG. 3, the molten glass lump 24 cut | disconnected from the molten glass flow 20 falls further to the downward Y1 side of a perpendicular direction.

-Press molding process (first step)-
In the first step, the first press mold and the second press mold in which the molten glass lump 24 shown in FIG. 3 is arranged to face each other in the direction intersecting the falling direction of the molten glass lump 24. To form a plate. Here, the first press mold and the second press mold are opposed to each other in a direction substantially perpendicular to the falling direction of the molten glass lump 24 so as to form an angle of 90 ° ± 1 °. It is particularly preferable that they are arranged so as to face each other in a direction orthogonal to the falling direction of the molten glass lump 24. Thus, by arranging the pair of press molds so as to face the falling direction of the molten glass lump 24, it is easier to press the molten glass lump 24 from both sides and form it into a plate shape.

  Further, the temperature of the press molding surfaces of the first press mold and the second press mold immediately before the first step is added to the strain point of the glass material constituting the molten glass lump 24 by 10 ° C. It is preferable that the temperature is not higher than the temperature obtained by adding 5 ° C. to the strain point of the glass material constituting the molten glass lump 24. By setting the temperature of the press-molded surface within the above-described range, it is possible to reliably suppress fusion of the molten glass lump 24 and the press-molded surface during press molding. The lower limit value of the temperature of the press molding surfaces of the first press mold and the second press mold immediately before the first step is not particularly limited, but the glass by rapid cooling of the molten glass lump 24 is not limited. It is above the strain point of the glass material constituting the molten glass lump 24 from a practical point of view, such as preventing cracking of the blank and preventing a significant decrease in stretchability of the molten glass lump 24 due to a sudden increase in viscosity during press molding. It is preferable.

  The absolute value of the difference between the temperature of the press molding surface of the first press mold and the temperature of the press molding surface of the second press mold immediately before the first step is 0 ° C. to 10 ° C. The temperature is preferably in the range of 0 ° C, more preferably in the range of 0 ° C to 5 ° C, and particularly preferably 0 ° C. In this case, it can suppress more reliably that the temperature difference in both surfaces of the plate glass formed in plate shape by the press of the molten glass lump 24 arises, and, as a result, flatness can be improved more.

  The absolute value of the in-plane temperature difference between the press forming surfaces of the first press mold and the second press mold immediately before the first step is performed is in the range of 0 ° C to 100 ° C. Is preferable, it is preferable to be within the range of 0 ° C to 50 ° C, and 0 ° C is particularly preferable. By setting the temperature distribution in the press molding surface within the above-described range, it becomes easier to stretch the molten glass lump 24 thinly and uniformly during press molding. As a result, even when a glass blank having a thinner plate thickness is manufactured, it is easier to obtain a glass blank that has excellent flatness and a small plate thickness deviation. The “in-plane temperature of the press-molded surface” means a temperature measured in the maximum region where the press-molded surface and the molten glass lump 24 stretched into a plate shape come into contact with each other during press molding.

  Next, the first step will be described more specifically with reference to the drawings. First, as shown in FIG. 4, the molten glass lump 24 shown in FIG. 3 includes a first press mold 50 and a second press that are opposed to each other in a direction orthogonal to the falling direction Y <b> 1 of the molten glass lump 24. It enters between the molds 60. Here, the first press molding die 50 and the second press molding die 60 before the press molding are arranged symmetrically with respect to the dropping direction Y1 and spaced apart from each other in the orthogonal direction. ing. Then, the molten glass lump 24 is pressed from both sides and press-molded in accordance with the timing when the molten glass lump 24 reaches the vicinity of the central portion in the vertical direction of the first press mold 50 and the second press mold 60. Therefore, the first press mold 50 is moved in the direction of the arrow X1 perpendicular to the drop direction Y1 and proceeds from the left side to the right side in the figure, and the second press mold 60 is It moves in the direction of the arrow X2 that is orthogonal to the falling direction Y1 and proceeds from the right side to the left side in the figure. The moving speed of the first press mold 50 in the arrow X1 direction and the moving speed of the second press mold 60 in the arrow X2 direction are set to be the same or substantially the same.

  Here, the press molds 50 and 60 include press mold main bodies 52 and 62 having a substantially disk shape, and guide members 54 and 64 arranged so as to surround the outer peripheral ends of the press mold main bodies 52 and 62. Have. 4 is a cross-sectional view, the guide members 54 and 64 are drawn so as to be positioned on both upper and lower sides of the press mold main bodies 52 and 62 in FIG. Further, the description of the drive member that moves the press mold 50 in the direction of the arrow X1 and moves the press mold 60 in the direction of the arrow X2 is omitted in the drawing.

  One surface of the press mold body 52, 62 is a press molding surface 52A, 62A. In FIG. 4, the first press mold 50 and the second press mold 60 are arranged to face each other so that the press molding surfaces 52 </ b> A and 62 </ b> A face each other. Further, the guide member 54 is provided with a guide surface 54A at a height position slightly protruding in the X1 direction with respect to the press molding surface 52A, and the guide member 64 is slightly in the X2 direction with respect to the press molding surface 62A. 64 A of guide surfaces are provided in the height position which protruded only. For this reason, in the press molding, the guide surface 54A and the guide surface 64A come into contact with each other, so that a gap is formed between the press molding surface 52A and the press molding surface 62A. For this reason, this gap thickness is the thickness of the molten glass lump 24 that is press-molded between the first press mold 50 and the second press mold 60, that is, the thickness of the glass blank. . Further, the press molding surfaces 52A and 62A are obtained by performing the first step so that the molten glass block 24 has a press molding surface 52A of the first press molding die 50 and a press molding surface 62A of the second press molding die 60. When the sheet is completely spread in the vertical direction and formed into a sheet glass, areas (melted glass stretching areas) S1 and S2 of the press molding surfaces 52A and 62A that are in contact with at least the sheet glass are , So as to form a substantially flat surface. In the example shown in FIG. 4, the entire surface of the press molding surface 52A including the molten glass stretching region S1 and the press molding surface 62A including the molten glass stretching region S2 is a flat surface having a normal and substantially zero curvature. Make up. In addition, the flat surface has only minute unevenness formed by performing normal flattening processing or mirror polishing processing when manufacturing a press mold, and is larger than these minute unevenness. There are no protrusions and / or recesses.

  The glass blank is produced by pressing the molten glass lump 24 by pressing it with the press molding surfaces 52A and 62A. For this reason, the surface roughness of the press molding surfaces 52A and 62A and the surface roughness of the main surface of the glass blank are substantially equal. The surface roughness (centerline average roughness Ra) of the main surface of the glass blank is 0.01 to 10 μm in performing scribing performed as a post-process described later and grinding using a diamond sheet. Since it is desirable to make it into the range, it is preferable that the surface roughness (centerline average roughness Ra) of the press-molded surface is also in the range of 0.01 to 10 μm.

  The molten glass lump 24 shown in FIG. 4 further falls downward and enters between the two press molding surfaces 52A and 62A. Then, as shown in FIG. 5, when the press molding surfaces 52A and 62A that are parallel to the falling direction Y1 reach the vicinity of the substantially central portion in the vertical direction, both side surfaces of the molten glass lump 24 are pressed to the press molding surfaces 52A and 52A, Contact 62A. At this time, it is preferable that the press molding surface 52A and the press molding surface 62A are brought into contact with the molten glass block 24 substantially simultaneously as shown in FIG. Here, “substantially simultaneously contact” means that the absolute value of the time difference between the time when the molten glass block and one press-molded surface are in contact with the point when the molten glass block and the other press-formed surface are in contact with each other. It means within 0.1 seconds. The absolute value of this time difference is more preferably within 0.05 seconds, and most preferably 0 seconds (simultaneous). For reference, in the vertical direct press method, the time required for the molten glass lump to contact the upper press forming surface after the molten glass lump contacts the upper press forming surface depends on the press forming conditions. Although it depends, it is generally about 1.5 seconds to 3 seconds.

  Further, as shown in FIG. 5, the press-molding surface 52A and the press-molding surface 62A are brought into contact with the molten glass block 24 substantially simultaneously, and at least during the press-molding step, the first press-molding process is performed. It is preferable that the temperature of the press molding surface 52A of the mold 50 and the temperature of the press molding surface 62A of the second press mold 60 are substantially the same. Accordingly, both sides of the molten glass block 24 being formed into a plate shape in the first step and the plate glass sandwiched between the pair of press molds 50 and 60 in the second step are always It will continue to be cooled symmetrically. In this case, there is almost no temperature difference between the two sides of the glass sheet after press molding, compared to the vertical direct press that press-molds the molten glass lump in which the viscosity distribution has occurred due to contact with the lower mold for a long time. In addition, it is possible to more reliably suppress a decrease in flatness due to a temperature difference between both surfaces.

  Here, “substantially the same” means that the absolute value of the temperature difference between the temperature of the press molding surface 52A of the first press mold 50 and the temperature of the press molding surface 62A of the second press mold 60 is the same. It means that it is within 10 ° C. The absolute value of this temperature difference is more preferably within 5 ° C, most preferably 0 ° C. In the case where there is a temperature distribution in the press molding surfaces 52A and 62A, the “temperature of the press molding surface” means a temperature near the center of the press molding surface. For reference, in the vertical direct press method, the absolute value of the temperature difference between the upper mold press surface and the lower mold press surface during the press molding of the molten glass lump is also determined by the press molding conditions. However, it is generally about 50 ° C to 100 ° C.

  In addition, considering the viewpoint of preventing the press position from fluctuating because it becomes difficult to press-mold due to the increase in viscosity of the molten glass lump 24 during the fall, or the drop speed becomes too high, It is preferable to select within the range of 1000 mm or less, more preferable to select within the range of 500 mm or less, further preferable to select within the range of 300 mm or less, and most preferable to select within the range of 200 mm or less. . The lower limit of the drop distance is not particularly limited, but is preferably 100 mm or more for practical use. The “fall distance” is the moment when the tip 22 is separated as the molten glass lump 24 as illustrated in FIG. 2, that is, from the position where the lower blade 30 and the upper blade 40 overlap in the vertical direction. As shown in FIG. 5, the position of the press molding start time (the moment of start of press molding), that is, the distance to the vicinity of the substantially central portion in the diametrical direction of the press molding surfaces 52A and 62A parallel to the drop direction Y1. means.

  Thereafter, as shown in FIG. 6, when the molten glass lump 24 is continuously pressed from both sides by the first press mold 50 and the second press mold 60, the molten glass lump 24 becomes the molten glass lump 24 and The press molding surfaces 52A and 62A are spread out with a uniform thickness around the position where they first contact. Then, as shown in FIG. 7, the press molding surfaces 52A and 62A are kept pressed by the first press molding die 50 and the second press molding die 60 until the guide surface 54A comes into contact with the guide surface 64A. In between, it is formed into a disk-shaped or substantially disk-shaped plate glass 26.

  Here, the plate-like glass 26 shown in FIG. 7 has substantially the same shape and thickness as the finally obtained glass blank. And the size and shape of both surfaces of the sheet glass 26 correspond with the size and shape of molten glass extending | stretching area | region S1, S2 (in FIG. 7, not shown). In addition, the time required for the guide surface 54A and the guide surface 64A shown in FIG. 7 to come into contact with each other from the state at the start of press molding shown in FIG. 5 (hereinafter, referred to as “press molding time” in some cases). .) Is preferably within 0.1 seconds from the viewpoint of thinning the molten glass lump 24. Further, when the press molding is performed, the guide surface 54A and the guide surface 64A are in contact with each other, so that it is easy to maintain the parallel state between the press molding surface 52A and the press molding surface 62A. In addition, although the minimum of press molding time is not specifically limited, It is preferable that it is 0.05 second or more practically.

  4 to 7, the press mold 50 includes a press mold body 52 and a guide member 54, and the press mold 60 has the same structure. Here, in the first step, the press mold main body 52 and the guide member 54 move in the direction of the arrow X1 simultaneously and integrally, and the press mold main body 62 and the guide member 64 simultaneously and integrally move. Move in the direction of arrow X2.

  Further, since the press molds 50 and 60 have guide members 54 and 64, respectively, when the guide member 54 and the guide member 64 are in contact with each other as shown in FIG. 7, the press mold surface 52A and the press mold surface 62A Are kept parallel. For this reason, as shown in FIGS. 4 to 6, in the process in which the press mold 50 moves in the arrow X1 direction and the press mold 60 moves in the arrow X2 direction, the press mold surface 52A and the press mold surface 62A Even if the parallel state cannot be maintained, it is easy to make the thickness deviation of the obtained glass blank very small. Therefore, in the drive device that drives the press molds 50 and 60, the press molding surface 52A and the press molding surface 62A always maintain an accurate parallel state in the series of processes shown in FIGS. The precise control ability to control is not required.

-Press molding process (second step)-
In the second step, as shown in FIG. 7, the first press mold 50 is in a state where the guide member 54 of the first press mold 50 and the guide member 64 of the second press mold 60 are in contact with each other. The press mold body 52 is driven to move in the direction of arrow X1, and the press mold body 62 of the second press mold 60 is driven to move in the direction of arrow X2. Thereby, the plate-like glass 26 is further pressed by the press mold main bodies 52 and 62.

  In addition, the glass sheet 26 immediately after finishing the first step has a high temperature and high fluidity (low viscosity). That is, the plate-like glass is very easily deformed and the flatness is easily deteriorated. For this reason, if the cooling of the plate glass 26 does not proceed so much and the second step is completed while maintaining a highly fluid state, the plate glass 26 is deformed after the second step is completed, and the glass blank The flatness of the may deteriorate. Therefore, the second step is preferably continued until the temperature of the sheet glass 26 becomes at least a temperature obtained by adding 10 ° C. to the strain point of the glass material constituting the sheet glass 26. That is, while maintaining the state immediately after the completion of the first step shown in FIG. It is preferable to continue pressing the glass sheet 26 with the press mold body 62. In this case, the cooling of the plate glass 26 sufficiently proceeds, the fluidity is lost, and the plate glass 26 is continuously pressed to a temperature range where the deformation is practically impossible. That is, the plate-like glass 26 can be solidified while maintaining the state in which the deformation of the plate-like glass 26 immediately after finishing the first step is suppressed. Therefore, the flatness of the produced glass blank can be made more excellent.

  Here, the duration of the second step is preferably controlled so that the flatness of the glass blank is 10 μm or less, and more preferably controlled so that the flatness of the glass blank is 4 μm or less. If the duration time of the second step is short, distortion occurs in the glass sheet 26 in the cooling process due to disturbance, and the distortion easily deteriorates the flatness of the glass blank. For this reason, the duration of the second step is changed, the flatness of the obtained glass blank is measured, and as a result, the duration of the second step is set so that the flatness is 10 μm or less, and the glass blank It is preferable to manufacture. However, if the duration of the second step is too long, the productivity is lowered. Therefore, the duration of the second step may be set in consideration of the flatness and productivity of the glass blank. From these viewpoints, the duration of the second step is specifically preferably in the range of 2 to 40 seconds, and preferably in the range of 2 to 30 seconds.

  Further, in order to control the flatness of the glass blank to 10 μm or less, in the second step, the plate glass is brought to a temperature range where the fluidity of the plate glass is lost and deformation is virtually impossible. It is particularly preferred to select the duration of the second step so as to continue pressing. In this case, the sheet glass 26 can be solidified while maintaining the state in which the deformation of the sheet glass 26 immediately after the first step is completed. Therefore, the flatness of the produced glass blank can be made more excellent. Here, the duration of the second step is such that the temperature of the sheet glass at the end of the second step is equal to or lower than the temperature obtained by adding 10 ° C. to the strain point of the glass material constituting the sheet glass. It is preferable to select, and it is more preferable to select the temperature to be equal to or lower than the temperature obtained by adding 5 ° C. to the strain point, and it is more preferable to select to be equal to or lower than the strain point. On the other hand, the lower limit temperature of the temperature of the sheet glass at the end of the second step is not particularly limited, but from the viewpoint of suppressing a decrease in productivity due to an increase in time required to perform the second step, practically, It is preferable that the strain point or higher. Therefore, it is preferable that the upper limit value of the duration in the second step is also selected from this viewpoint.

  In addition, immediately after the start of the first step in which the press molding surfaces 52A and 62A are in contact with the molten glass lump 24, the press molding surface 52A and the press molding surface 62A The temperature of the glass (molten glass lump 24 and plate-like glass 26) located between is generally about 1200 ± 50 ° C. to about 480 ° C. ± 20 ° C., although it depends on the glass material used for press molding. Will drop significantly. Therefore, in the second step, thermal contraction in the radial direction of the sheet glass 26 proceeds with such a significant decrease in temperature. Such heat shrinkage reaches a temperature range in which the temperature of the sheet glass 26 is lower, in particular, a temperature range equal to or lower than the temperature obtained by adding 10 ° C. to the strain point of the glass material constituting the sheet glass 26. It becomes more remarkable when the second step is continued. On the other hand, in the second step, the press-molded surfaces 52A and 62A that are in contact with both surfaces of the plate glass 26 continue to absorb the heat of the plate glass 26 and thermally expand in the plane direction, or the plate glass It is considered that the thermal expansion in the plane direction is stopped or the heat is gradually contracted by absorbing sufficient heat from 26.

  That is, in the second step, a difference occurs in the degree of thermal expansion / contraction between both members between the both surfaces of the sheet glass 26 and the press-molding surfaces 52A, 62A. For this reason, in the second step, on both surfaces of the sheet glass 26 that is being thermally contracted, the force to extend in the radial direction of the sheet glass 26 by the press molding surfaces 52A and 62A, that is, heat A force in the direction opposite to the contraction acts. However, in the second step, the fluidity of the sheet glass 26 decreases as the second step proceeds, and therefore, if an excessive stress acts on the sheet glass 26, the sheet glass 26 is liable to be brittle fractured. . For this reason, if a force in the direction opposite to the thermal shrinkage always acts on both surfaces of the plate glass 26, excessive stress acts in the plane direction of the plate glass 26 and the plate glass 26 is broken. Become.

  In order to prevent such cracking of the glass sheet 26, (1) As a material constituting the press molds 50 and 60, a material having a thermal expansion coefficient close to that of the glass material constituting the glass sheet 26 is used. (2) In the second step, it is conceivable that the temperature of the sheet glass 26 and the temperature of the press molding surfaces 52A and 62A are synchronized and cooled. However, since the second step involves a significant temperature change, it is necessary to make the cooling rate very small when performing the cooling as described above. However, in this case, since the time required for performing the second step is significantly increased, the mass productivity may be significantly reduced and the practicality may be lacking.

  Considering the points described above, it is preferable to decrease the press pressure with time in the second step in order to more reliably prevent cracking of the glass sheet 26 in the second step. In this case, the friction coefficient between the both surfaces of the sheet glass 26 and the press-molded surfaces 52A and 62A decreases due to the decrease in the press pressure. As a result, slip occurs between the both surfaces of the sheet glass 26 and the press molding surfaces 52A and 62A, and a force in the opposite direction to the heat shrinkage that causes cracking acts on both surfaces of the sheet glass 26. It becomes easy to block. Here, “the press pressure decreases with time” is not only the case where the press pressure decreases with the passage of time in the second step, but also the press pressure temporarily changes with the passage of time. Even when increasing or maintaining a constant value, the case where the slope becomes negative when the change of the press pressure with respect to time is approximated by a linear equation is included. Further, the press pressure may be decreased stepwise with the passage of time, or may be continuously decreased with the passage of time.

  When the press pressure is decreased step by step with time, the temperature of the sheet glass 26 sandwiched between the first press mold 50 and the second press mold 60 is reduced. It is preferable that the glass material constituting the plate-like glass 26 is reduced when it falls within the range of the yield point ± 30 ° C. Thereby, the crack of the sheet glass 26 can be more effectively suppressed by a relatively simple operation of the pressing pressure. In this case, from the viewpoint of achieving both balanced suppression of cracking of the sheet glass 26 and suppression of deterioration of flatness in a well-balanced manner, the pressing pressure is 1% to 60 after the reduction, assuming that the pressure before the reduction is 100%. It is preferable to be in the range of about%.

-Extraction process-
After passing through the second step, the first press mold 50 and the second press mold 60 are separated and sandwiched between the first press mold 50 and the second press mold 60. The extraction process which takes out the plate-shaped glass 26 which performed is performed. This extraction step can be performed, for example, as described below. First, as shown in FIG. 8, the first press mold 50 is moved in the arrow X2 direction so as to separate the first press mold 50 and the second press mold 60 from each other. The press mold 60 is moved in the arrow X1 direction. Thereby, the press-molding surface 62A and the sheet glass 26 are released. Next, as shown in FIG. 9, the press molding surface 52 </ b> A and the plate glass 26 are released from the mold, and the plate glass 26 is dropped to the lower side Y <b> 1 in the vertical direction and taken out. In addition, when releasing the press molding surface 52A and the plate-like glass 26, it is possible to release the plate-like glass 26 by applying a force from the outer peripheral direction of the plate-like glass 26. In this case, the sheet glass 26 can be taken out without applying a large force. At the time of taking out, the press molding surface 52A and the plate glass 26 may be released, and then the press molding surface 62A and the plate glass 26 may be released. Finally, the extracted plate-like glass 26 is annealed as necessary to reduce and remove the distortion, thereby obtaining a base material for processing the magnetic recording medium glass substrate, that is, a glass blank.

-Glass blank-
The flatness of the glass blank obtained by the glass blank manufacturing method of the present embodiment described above can be, for example, 10 μm or less, and can be extremely easily set to 4 μm or less. The flatness is preferably 4 μm or less from the viewpoint of omitting or shortening post-processes performed mainly for improving flatness such as a lapping process.

-Press mold-
The press mold 50 used in the glass blank manufacturing method of the present embodiment includes at least a press mold body 52 and a guide member 54. The press mold 60 also has at least a press mold body 62 and a guide member 64 and has the same structure as the press mold 50. Hereinafter, the press mold 50 will be described as an example. First, the press mold 50 is configured as a separate member from the press mold body 52 and the guide member 54. For this reason, in the first step, the press mold main body 52 and the guide member 54 can be integrally driven to be pushed toward the press mold 60 arranged opposite to each other, and in the second step. It is possible to drive only the press mold main body 52 relative to the guide member 54 so as to be pushed out relatively to the press mold 60 side disposed opposite to the guide member 54. And since the press mold 50 has the structure and function as described above, it is possible to further reduce the thickness deviation and flatness of the glass blank.

  If attention is paid only to reducing the thickness deviation, a press mold in which the press mold main body 52 and the guide member 54 are integrally formed can be used. However, in this type of press mold, it is impossible to drive only the press mold body 52 relative to the guide member 54 so as to be pushed out relatively to the press mold 60 side arranged oppositely. For this reason, even if the first step is finished and the state where the guide member 54 and the guide member 64 are kept in contact with each other, both surfaces of the sheet glass 26 are always closely adhered to each other by the press molding surfaces 52A and 62A. I can't support it. Therefore, the flatness of the glass blank tends to deteriorate.

  If attention is paid only to reducing the flatness, a press mold without the guide member 54 (a guide member-less mold) can be used. In this type of press mold, even after the molten glass lump 24 is press-molded into a sheet glass 26, both surfaces of the sheet glass 26 are always closely supported by the press-molded surfaces 52A and 62A and supported. be able to. However, since the guide members 54 and 64 do not exist, it is difficult to perform press molding in a state where the press molding surface 52A and the press molding surface 62A are accurately kept in parallel unless the press mold is driven very precisely. For this reason, the plate | board thickness deviation of a glass blank becomes easy to deteriorate.

  Considering the points described above, the press mold 50 (and the press mold 60) having at least the guide member 54 and the press mold main body 52 which are configured as separate members have a thickness variation and flatness of the glass blank. It is extremely advantageous in that both of the properties can be improved in a balanced manner.

  The material constituting the press molds 50 and 60 is preferably a metal or an alloy in consideration of heat resistance, workability, and durability. In this case, considering the temperature of the molten glass, the heat resistance temperature of the metal or alloy constituting the press molds 50 and 60 is preferably 1000 ° C. or higher, and more preferably 1100 ° C. or higher. Specifically, the material constituting the press molds 50 and 60 is preferably spheroidal graphite cast iron (FCD), alloy tool steel (such as SKD61), high speed steel (SKH), cemented carbide, colmonoy, stellite, and the like. In press molding, the press molds 50 and 60 may be cooled using a cooling medium such as water or air to suppress an increase in the temperature of the press molds 50 and 60. Further, in order to make the temperature distribution in the press molding surfaces 52A, 62A uniform, the vicinity of the center of the press molding surfaces 52A, 62A is cooled using a cooling medium, and / or the press mold 50 is used. Further, a heating member such as a heater may be disposed on the outer peripheral side of 60 to heat the outer edge side of the press molding surfaces 52A and 62A.

  Moreover, the area | region (molten glass extending | stretching area | region S1, S2) which contacts at least the sheet glass 26 of the press molding surfaces 52A and 62A of the 1st press mold 50 and the 2nd press mold 60 is a glass blank, for example. The surface may have a surface on which a large uneven portion such as a convex portion for forming a V-shaped groove having a depth of about 1/3 to 1/4 of the plate thickness may be formed. A surface is preferred. Note that the entire pressing surfaces 52A and 62A may be substantially flat surfaces. This is because when a large V-shaped groove is formed in the glass blank, crack defects presumed to be caused by stress concentration in the V-shaped groove portion are likely to occur. In addition to this, when large irregularities are formed in the molten glass stretching regions S1 and S2, thermal contraction in the radial direction of the sheet glass 26 in the second step is hindered. For this reason, excessive stress is generated in the planar direction of the sheet glass 26, and the sheet glass 26 is easily broken.

  Here, the “substantially flat surface” means a surface having a very small curvature such as a slight convex surface or a concave surface in addition to a normal flat surface having substantially zero curvature. In addition, it is naturally allowed that the “substantially flat surface” has minute irregularities formed by performing a normal flattening process or a mirror polishing process when manufacturing a press mold. However, larger protrusions and / or recesses may be provided as necessary as compared with the minute unevenness.

  Here, the convex portion larger than the minute irregularities is substantially point-like having a height of 20 μm or less with a small possibility of causing deterioration of flow resistance or promoting partial cooling of the molten glass lump. And / or a substantially linear protrusion is acceptable. The height is preferably 10 μm or less, and more preferably 5 μm or less. Further, the convex portion larger than the minute unevenness is not substantially punctiform and substantially linear, and the convex shape is a trapezoidal shape with a minimum width of the top surface of several millimeters or more, or In the case of a dome-shaped convex part having the same height and size as this trapezoidal convex part, the flow resistance is deteriorated as described above, or partial cooling of the molten glass lump is promoted. Therefore, the height is allowed to be 50 μm or less. The height is preferably 30 μm or less, and more preferably 10 μm or less. Further, from the viewpoint of suppressing the occurrence of cracks due to stress concentration at the intersection of the bottom surface and side surface of the trapezoidal convex portion, the side surface of the trapezoidal convex portion has an inclination angle of 0.5 with respect to the top surface. It is preferable to form a plane that forms an angle of less than or equal to a degree, or a curved surface with this plane as a concave surface. The angle is more preferably 0.1 degrees or less.

  Further, the larger recess compared with the minute unevenness is substantially a spot having a depth of 20 μm or less and / or so as not to cause deterioration of the fluidity of the molten glass flowing into the recess during press molding. Or if it is a substantially linear recessed part, it will be accept | permitted. The depth is preferably 10 μm or less, and more preferably 5 μm or less. Further, the concave portion larger than the minute unevenness is not substantially punctiform or substantially linear, and the concave portion having an inverted trapezoidal shape with a minimum width of the top surface of several millimeters or more, or In the case of an inverted dome-shaped recess having the same height and size as the inverted trapezoidal recess, the possibility of causing the above-described deterioration in fluidity is reduced, so the depth is 50 μm or less. If that is acceptable. The depth is preferably 30 μm or less, and more preferably 10 μm or less. Further, from the viewpoint of suppressing the occurrence of cracks due to stress concentration at the intersection between the bottom surface and the side surface of the trapezoidal convex portion, the inclination angle of the side surface of the trapezoidal convex portion is 0.5 degrees with respect to the bottom surface. It is preferable to form a plane having the following angle, or a curved surface with this plane as a concave surface. The angle is more preferably 0.1 degrees or less.

  4 to 9, the press mold 50 (and the press mold 60) includes at least a press mold body 52 and a guide member 54, and includes a first step and a second step. As long as it can be implemented, the specific structure is not particularly limited, but in addition to the press mold main body 52 and the guide member 54, the first extrusion member and the second extrusion member are further provided. It is preferable. Here, the first extruding member is a side of the press mold 60 in which the press mold main body 52 and the guide member 54 are orthogonal to the press mold surface 52A and are opposed to the press mold surface 52A. And at least a function of extruding at the same time. Further, the second extrusion member is pressed by the first extrusion member after the guide member 54 and a part of the press molding die 60 (the guide member 64) arranged to face the press molding surface 52A come into contact with each other. The main body 52 has at least a function of pushing out the main body 52 in a direction orthogonal to the press molding surface 52A and to the side of the press molding die 60 disposed opposite to the press molding surface 52A.

  FIG. 10 is a schematic cross-sectional view showing an example of a press mold used in the method for manufacturing a glass blank for a magnetic recording medium glass substrate of the present embodiment. Specifically, the press molds 50 and 60 are more specific. It is a figure which shows an example of a simple structure. In FIG. 10, the same components as those shown in FIGS. 4 to 9 are denoted by the same reference numerals. 10 is a view corresponding to the press mold 50, but the press mold 60 can employ the same structure. Here, the main part of the press mold 50 </ b> S includes a press mold main body 52, a guide member 54, a first push member 56, and a second push member 58. The central axes of the respective members (in the drawing, the alternate long and short dash line X) coincide with each other, and the central axes substantially coincide with the horizontal direction.

  Here, the press-molding die main body 52 is composed of a cylindrical body having one end surface forming a circular press-molding surface 52A. In addition, although the shape of the press-molding die main body 52 is a columnar shape in the example illustrated in FIG. 10, the shape is not particularly limited as long as it is a substantially columnar shape. In the example shown in FIG. 10, the press-molding surface 52A is a substantially flat surface.

  The guide member 54 has a length in the axial direction X that is longer than the length in the axial direction X of the press-molding die main body 52 made of a cylindrical body, and accommodates the press-molding die main body 52 on the inner peripheral side. When extruded by 56, one end surface (guide surface 54A) is composed of a cylindrical body that comes into contact with a guide member (not shown in the drawing) constituting the other press mold. Here, the difference between the length of the guide member 54 and the length of the press mold body 52 in the axial direction X, in other words, the height difference H between the guide surface 54A and the press molding surface 52A in the axial direction X is the glass to be produced. This corresponds to approximately half the blank thickness. The shape of the guide member 54 is cylindrical, but the shape is not particularly limited as long as it is cylindrical.

  The 1st extrusion member 56 is comprised from the disk shaped member. Here, one surface (extruded surface 56A) of the disc-shaped first extruding member 56 is the other end surface (extruded surface 52B) of the press mold main body 52 and the other end surface (extruded surface) of the guide member 54. It consists of a flat surface in contact with the surface 54B). In addition, a through hole 56 </ b> H that penetrates in the thickness direction of the first extrusion member 56 is provided in a part of a region facing the extruded surface 52 </ b> B of the press mold main body 52. The surface 56B opposite to the extrusion surface 56A is connected to a first drive device (not shown). For this reason, during press molding, the first driving device simultaneously pushes the press mold body 52 and the guide member 54 through the first pusher member 56 in the axial direction X in the drawing. It can be extruded from the side where the member 56 is disposed to the side where the press mold main body 52 and the guide member 54 are disposed.

  In the example shown in FIG. 10, the shape of the first pushing member 56 is a disc shape, but the shape is not particularly limited as long as it is substantially plate-shaped. The through hole 56H is provided as a hole having a circular opening along the center axis X of the press mold main body 52 and the first extrusion member 56. However, the extruded surface 52B of the press mold main body 52 is provided. If it is a part of the area | region which opposes, the arbitrary number of through-holes 56H can be provided in the arbitrary positions of the 1st extrusion member 56. FIG. Moreover, the opening shape of the through-hole 56H can also be selected suitably. However, it is particularly preferable that the through hole 56H is provided point-symmetrically with respect to the central axis X of the press mold main body 52.

  The second pushing member 58 is disposed in the through hole 56 </ b> H, and is composed of a rod-like member connected to the pushed surface 52 </ b> B side of the press mold main body 52. In the example shown in FIG. 10, the second pushing member 58 has a cylindrical bar shape, but the shape is not particularly limited as long as the press mold main body 52 can be moved in the X-axis direction. Note that the end opposite to the one end connected to the extruded surface 52B side of the second pushing member 58 is connected to a second drive device (not shown). For this reason, during press molding, only the press mold main body 52 is pressed from the side where the second extruding member 58 is disposed by the second driving device via the second extruding member 58. It can be extruded along the axial direction X to the side where 52 is arranged.

  In press forming, in order to make it easy to stretch the molten glass lump 24 thinly and uniformly, it is preferable that the temperature distribution in the surface of the press forming surface 52A can be controlled to be uniform. For this purpose, (1) a heating member for heating the vicinity of the outer edge side of the press molding surface 52A is provided, and / or (2) the inside of the press mold main body 52 and on the press molding surface 52A side. A flow path through which the cooling medium flows can be provided at least near the center.

  Here, as the heating member, for example, a cylindrical heater is arranged on the outer peripheral side of the guide member 54, or rod-shaped heaters parallel to the axial direction X are equally spaced along the circumferential direction of the guide member 54. Can be arranged. These heaters may be built in the guide member 54 or may be arranged so as to be embedded on the outer peripheral surface side of the press mold main body 52. As the cooling liquid, a liquid such as water, a gas such as air, a gas sprayed and dispersed, and the like can be used.

  Further, as the press molds 50 and 60, a press mold illustrated in FIG. 11 can be used. FIG. 11 is a schematic cross-sectional view showing another example of a press mold used in the method for manufacturing a glass blank for a magnetic recording medium glass substrate of the present embodiment. In FIG. 11, those having substantially the same or similar functions as those shown in FIG. 10 are denoted by the same reference numerals.

  Here, the main part of the press mold 100 shown in FIG. 11 is composed of a press mold main body 52, a guide member 54, a first push member 56, and a second push member 58. Yes. The central axes of the respective members (in the drawing, the alternate long and short dash line X) coincide with each other, and the central axes substantially coincide with the horizontal direction. The press mold 100 shown in FIG. 11 and the press mold 50S shown in FIG. 10 include a press mold body 52, a guide member 54, a first push member 56, and a second push member 58. Although common, the following points are largely different. That is, compared with the press mold 50S shown in FIG. 10, in the press mold 100 shown in FIG. 11, (1) the press mold main body 52 and the guide member 54 are the outer peripheral surface of the press mold main body 52 and the guide member 54. (2) The press-molding die main body 52 and the first extrusion member 56 are arranged such that the surface to be extruded 52B of the press-molding die main body 52 and the first extrusion The member 56 is disposed so as to be substantially spaced from the extrusion surface 56A. (3) A support member 70 is disposed between the extruded surface 52B and the extruded surface 56A along the outer peripheral side of the extruded surface 52B. Is arranged.

  Here, the press-molding die main body 52 is composed of a disk-shaped member whose one end surface forms a circular press-molding surface 52A. In addition, although the shape of the press mold main body 52 is a disk shape in the example shown in FIG. 10, the shape is not particularly limited as long as it is substantially a disk shape. In the example shown in FIG. 11, the press-molding surface 52A is a substantially flat surface. Further, the support member 70 is fixed to one of the extruded surface 52B and the extruded surface 56A and can be separated from the other surface. As the support member 70, for example, a ring-shaped member can be used.

  In the press mold 100 shown in FIG. 11, the guide member 54 can be pushed out to the other press mold side disposed opposite to the press mold 100 by the first extruding member 56. At the same time, the press mold body 52 is also pushed out to the other press mold side facing the press mold 100 via the support member 70. Further, only the press mold main body 52 can be pushed out to the other press mold side facing the press mold 100 by the second pushing member 58. In the example shown in FIG. 10, during press molding, the press mold main body 52 is subjected to a pressing force in the vicinity of (1) the central portion of the surface 52B to be extruded or (2) the vicinity of the outer edge portion. For this reason, the thickness, material, strength, etc. of the press mold main body 52 so that the press mold main body 52 will not bend even if a pressing force is applied to any position shown in the above (1) and (2). It is preferable to select a pressing condition such as the strength of the supporting member 70 or a pressing pressure.

  In addition, although the example shown in FIGS. 4-9 is shown about the 2nd press process, when implementing a 1st press process, only one press mold of a pair of press molds is carried out. For example, the press mold 50S shown in FIG. 10 or the press mold 100 shown in FIG. 11 may be used. In this case, as the other press mold, for example, a press mold which will be described later (for example, shown in FIG. 13 which will be described later) substantially composed of only a press mold main body portion such as a simple disk-shaped member or a columnar member. A press mold 210) can be used. In this case, for example, in the first step, a part of the other press mold (for example, a press molding surface) and the guide surface 54A are in contact with each other, and in the second step, a part of the other press mold and the guide surface. With the contact with 54A, the press mold main body 52 is further pushed out to the other press mold side.

-Glass material-
The glass material used in the method for producing a glass blank of the present embodiment has physical properties suitable as a magnetic recording medium glass substrate, in particular, a high thermal expansion coefficient, high rigidity, heat resistance, etc., and a horizontal direct press. If it is easy to press-form into a plate shape, there is no particular limitation. The thermal expansion coefficient is desirably close to the thermal expansion coefficient of the holder that holds the magnetic recording medium. Specifically, the average linear expansion coefficient at 100 to 300 ° C. is preferably 70 × 10 ⁻⁷ / ° C. or more, more preferably 75 × 10 ⁻⁷ / ° C. or more, and 80 × 10 ⁻⁷ / ° C. It is more preferable that the temperature is not lower than ° C., and it is even more preferable that the temperature is 85 × 10 ⁻⁷ / ° C. or higher. The upper limit of the average linear expansion coefficient is not particularly limited, but is practically preferably 120 × 10 ⁻⁷ / ° C. or less. A glass material having high rigidity is desired from the viewpoint of reducing the deflection generated during high-speed rotation of the magnetic recording medium. Specifically, the Young's modulus is preferably 70 GPa or more, more preferably 75 GPa or more, and 80 GPa. More preferably, it is more preferably 85 GPa or more. The upper limit of the Young's modulus is not particularly limited, but is practically preferably 120 GPa or less. Furthermore, since a substrate can be processed at a high temperature during the production process of a magnetic recording medium by using a glass material having excellent heat resistance, the glass transition temperature of the glass material is preferably 600 ° C. or higher, and 610 ° C. The above is more preferable, 620 ° C. or higher is further preferable, and 630 ° C. or higher is more preferable. The upper limit of the glass transition temperature is not particularly limited, but is preferably 780 ° C. or lower from a practical viewpoint such as suppressing the temperature during press molding from becoming high. Use of a glass material having a high thermal expansion coefficient, high rigidity, and heat resistance is effective in obtaining a glass substrate suitable for a magnetic recording medium having a high recording density.

  As the composition of the glass material, a composition that can easily realize physical properties suitable as a magnetic recording medium glass substrate can be appropriately selected. For example, the glass composition of a conventional glass material for vertical direct press can be appropriately selected, but an aluminosilicate glass Is preferably selected. In addition, as an aluminosilicate glass, since it is easy to make heat resistance, high rigidity, and a high thermal expansion coefficient compatible with balance, especially the composition shown below is especially preferable.

That is, the glass composition of this glass (hereinafter referred to as glass composition 1) is
In mol% display,
The SiO ₂ 50% ~75%,
Al ₂ O ₃ from 0% to 5%,
Li ₂ O 0% -3%,
ZnO from 0% to 5%,
3% to 15% in total of at least one component selected from Na ₂ O and _{K 2} O,
A total of 14% to 35% of at least one component selected from MgO, CaO, SrO and BaO, and
₂ to 9% in total of at least one component selected from ZrO ₂ , TiO ₂ , La ₂ O ₃ , Y ₂ O ₃ , Yb ₂ O ₃ , Ta ₂ O ₅ , Nb ₂ O ₅ and HfO ₂ ;
Including,
The molar ratio {(MgO + CaO) / (MgO + CaO + SrO + BaO)} is in the range of 0.8 to 1, and the molar ratio {Al ₂ O ₃ / (MgO + CaO)} is in the range of 0 to 0.30.

The preferable range of the average linear expansion coefficient of the glass composition 1 at 100 to 300 ° C. is 70 × 10 ⁻⁷ / ° C. or more, the preferable range of the glass transition temperature is 630 ° C. or more, and the preferable range of the Young's modulus is 80 GPa or more. Glass composition 1 is suitable as a material for an energy-assisted magnetic recording medium glass substrate using a high Ku magnetic material.

  Further, as a glass material having a high thermal expansion coefficient, excellent acid resistance and alkali resistance, little alkali elution from the substrate surface, and suitable for chemical strengthening, one having the following glass composition (referred to as glass composition 2). Can show.

That is, the glass composition 2 is
In mol% display,
SiO ₂ and Al ₂ O ₃ in total 70% to 85% (however, the content of SiO ₂ is 50% or more, the content of Al ₂ O ₃ is 3% or more),
Li ₂ O, Na ₂ O and K ₂ O in total 10% or more,
1% to 6% in total of MgO and CaO (however, the content of CaO is larger than the content of MgO),
ZrO ₂ , TiO ₂ , La ₂ O ₃ , Y ₂ O ₃ , Yb ₂ O ₃ , Ta ₂ O ₅ , Nb ₂ O ₅ and HfO ₂ in total exceeding 0% and 4% or less,
It is a composition containing.

(Method for manufacturing magnetic recording medium glass substrate)
The manufacturing method of the magnetic recording medium glass substrate of this embodiment manufactures a magnetic recording medium glass substrate at least through a polishing step of polishing the main surface of the glass blank produced by the glass blank manufacturing method of this embodiment. It is characterized by. Specific examples of the respective steps when processing a glass blank to form a magnetic recording medium glass substrate will be described in more detail below.

  First, scribing is performed on a glass blank obtained by press molding. Scribe is a glass blank that is cut into two concentric circles (inner concentric circle and outer concentric circle) with a scriber made of super steel alloy or diamond particles on the surface of the glass blank in order to make the molded glass blank into a ring shape of a predetermined size This refers to providing a line (linear scratch). The glass blank scribed in two concentric shapes is partially heated, and the difference in thermal expansion of the glass removes the outer portion of the outer concentric circle and the inner portion of the inner concentric circle. As a result, a perfect circular disk-shaped glass is obtained.

  When scribing, if the roughness of the main surface of the glass blank is 1 μm or less, a cutting line can be suitably provided using a scriber. In addition, when the roughness of the main surface of a glass blank exceeds 1 micrometer, a scriber may not follow surface unevenness | corrugation and it may become difficult to provide a cutting line uniformly. In this case, scribing is performed after the main surface of the glass blank is smoothed.

  Next, shape processing of the scribed glass is performed. Shape processing includes chamfering (chamfering of the outer peripheral end and the inner peripheral end). In chamfering, chamfering is performed on the outer peripheral end and inner peripheral end of the ring-shaped glass with a diamond grindstone.

  Next, the end surface of the disk-shaped glass is polished. In the end surface polishing, the inner peripheral side end surface and the outer peripheral side end surface of the glass are mirror finished by brush polishing. At this time, a slurry containing fine particles such as cerium oxide as free abrasive grains is used. Preventing the occurrence of ion precipitation that causes corrosion such as sodium and potassium by removing contamination such as dirt, damage or scratches attached to the end surface of the glass by end face polishing Can do.

  Next, 1st grinding | polishing is given to the main surface of disk-shaped glass. The first polishing is intended to remove scratches and distortions remaining on the main surface. The machining allowance by the first polishing is, for example, about several μm to 10 μm. Since it is not necessary to perform a grinding process with a large machining allowance, the glass is not scratched or distorted due to the grinding process. Therefore, the machining allowance in the first polishing process is small.

  In the first polishing step and the second polishing step described later, a double-side polishing apparatus is used. The double-side polishing apparatus is an apparatus that performs polishing by using a polishing pad and relatively moving a disk-shaped glass and a polishing pad. The double-side polishing apparatus includes a polishing carrier mounting portion having an internal gear and a sun gear that are driven to rotate at a predetermined rotation ratio, and an upper surface plate and a lower surface plate that are driven to rotate reversely with respect to the polishing carrier mounting portion. Have A polishing pad, which will be described later, is attached to the surfaces of the upper and lower surface plates facing the disk-shaped glass. The polishing carrier mounted so as to mesh with the internal gear and the sun gear revolves around the sun gear while rotating around the sun gear.

  Each of the polishing carriers holds a plurality of disc-shaped glasses. The upper surface plate is movable in the vertical direction, and presses the polishing pad against the main surfaces of the front and back surfaces of the disk-shaped glass. Then, while supplying a slurry (polishing liquid) containing abrasive grains (polishing material), the planetary gear motion of the polishing carrier and the upper surface plate and the lower surface plate rotate reversely to each other, so that the disk-shaped glass and the polishing pad The main surfaces of the front and back surfaces of the disk-shaped glass are polished. In the first polishing step, for example, a hard resin polisher is used as the polishing pad, and for example, cerium oxide abrasive grains are used as the abrasive.

  Next, the disk-shaped glass after the first polishing is chemically strengthened. For example, a molten salt of potassium nitrate can be used as the chemical strengthening solution. In chemical strengthening, the chemical strengthening solution is heated to, for example, 300 ° C. to 400 ° C., and the cleaned glass is preheated to, for example, 200 ° C. to 300 ° C., and then the glass is placed in the chemical strengthening solution, for example, 3 hours to 4 hours. Soaked. The immersion is preferably performed in a state of being housed in a holder so that a plurality of glasses are held at the end faces so that both main surfaces of the glass are chemically strengthened.

  In this way, by immersing the glass in the chemical strengthening solution, sodium ions on the surface layer of the glass are respectively replaced with potassium ions having a relatively large ionic radius in the chemical strengthening solution, and the thickness of about 50 to 200 μm. A compressive stress layer is formed. As a result, the glass is strengthened and has good impact resistance. Note that the chemically strengthened glass is washed. For example, after washing with sulfuric acid, it is washed with pure water, IPA (isopropyl alcohol) or the like.

  Next, second polishing is performed on the chemically strengthened and sufficiently cleaned glass. The machining allowance by the second polishing is, for example, about 1 μm.

  The second polishing is intended to finish the main surface into a mirror surface. In the second polishing step, as with the first polishing step, the disc-shaped glass is polished using a double-side polishing apparatus. The polishing abrasive grains contained in the polishing liquid (slurry) to be used and the composition of the polishing pad Is different. In the second polishing step, the grain size of the abrasive grains to be used is made smaller than in the first polishing step, and the hardness of the polishing pad is made softer. For example, in the second polishing process, for example, a soft foamed resin polisher is used as the polishing pad, and as the abrasive, for example, cerium oxide abrasive grains that are finer than the cerium oxide abrasive grains used in the first polishing process are used.

  The disk-shaped glass polished in the second polishing process is washed again. In cleaning, a neutral detergent, pure water, and IPA are used. By the second polishing, a magnetic disk glass substrate having a main surface flatness of 4 μm or less and a main surface roughness of 0.2 nm or less is obtained. Thereafter, each layer such as a magnetic layer is formed on the glass substrate for magnetic disk to produce a magnetic disk.

  In addition, although a chemical strengthening process is performed between a 1st grinding | polishing process and a 2nd grinding | polishing process, it is not limited to this order. As long as a 2nd grinding | polishing process is performed after a 1st grinding | polishing process, a chemical strengthening process can be arrange | positioned suitably. For example, the order of the first polishing process, the second polishing process, and the chemical strengthening process (hereinafter, process order 1) may be used. However, in the process order 1, since the surface unevenness | corrugation which may arise by a chemical strengthening process is not removed, the process order of a 1st grinding | polishing process, a chemical strengthening process, and a 2nd grinding | polishing process is more preferable.

  When manufacturing the magnetic recording medium glass substrate, the flatness of the glass blank used for processing and the flatness of the produced magnetic recording medium glass substrate can be made substantially the same. As flatness required for a magnetic recording medium glass substrate, for example, a glass substrate of 2.5 inches has recently been required to have a flatness of 10 μm or less. This is because it can be easily achieved by the glass blank produced by the method. Here, “the flatness of the glass blank used for processing and the flatness of the produced magnetic recording medium glass substrate are substantially the same” is based on the flatness of the required magnetic recording medium glass substrate ( 100%) means that the flatness of the glass blank is 105% or less.

  When the flatness of the glass blank used for processing and the flatness of the produced magnetic recording medium glass substrate are substantially the same, improvement of flatness such as a lapping process is performed as one of the main purposes. The process can be omitted.

(Method of manufacturing magnetic recording medium)
The magnetic recording medium manufacturing method of the present embodiment has undergone at least a magnetic recording layer forming step of forming a magnetic recording layer on the magnetic recording medium glass substrate produced by the magnetic recording medium glass substrate manufacturing method of the present embodiment, A magnetic recording medium is manufactured.

  Magnetic recording media are called magnetic disks, hard disks, etc., internal storage devices (such as fixed disks) such as desktop PCs, server computers, notebook PCs, and mobile PCs, and portable recording and playback that records and plays back images and / or audio. It is suitable for an internal storage device of a device, an in-vehicle audio recording / reproducing device, and the like.

For example, at least an adhesion layer, an underlayer, a magnetic layer (magnetic recording layer), a protective layer, and a lubricating layer are stacked on the main surface of a magnetic recording medium glass substrate in order from the side closer to the main surface. It can be configured. For example, a magnetic recording medium glass substrate is introduced into a depressurized film forming apparatus, and an adhesion layer to a magnetic layer are sequentially formed on the main surface of the magnetic recording medium glass substrate in an Ar atmosphere by a DC magnetron sputtering method. . For example, CrTi can be used as the adhesion layer, and CrRu can be used as the underlayer. After the above film formation, a magnetic recording medium can be formed by forming a protective layer using C ₂ H ₄ gas, for example, by the CVD method, and performing nitriding treatment in which nitrogen is introduced into the surface in the same chamber. it can. Thereafter, for example, PFPE (polyfluoropolyether) is applied on the protective layer by a dip coating method, whereby the lubricating layer can be formed.

  The size of the magnetic recording medium is not particularly limited, but since the magnetic recording medium glass substrate is made of a glass material with excellent impact resistance, it is convenient to carry and may be exposed to external impacts. A size of 2.5 inches or less is preferred.

  EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to the following examples.

<< Production of Glass Blank >>
In each example and comparative example, several hundred or more glass blanks (diameter: about 75 mm, thickness: about 0.9 mm) for producing a 2.5 inch size magnetic recording medium glass substrate were continuously produced.

(Example A1)
In the process shown in FIGS. 1 to 9, a molten glass lump forming step, a press molding step (first step and second step), and an extraction step were carried out to produce a glass blank. The viscosity of the molten glass flowing out from the glass outlet 12 is adjusted to 700 dPa · s, and the first press mold 50 and the second press mold 60 are orthogonal to the dropping direction of the molten glass lump 24. The drop distance was set to 150 mm.

Here, the main physical property value and composition of the glass material used for preparation of the glass blank are as follows.
Glass transition temperature: 495 ° C
・ Bend point: 550 ℃
・ Strain point: 490 ℃
Composition: composition corresponding to the glass composition 2

  Moreover, the concrete implementation conditions of a 1st step and a 2nd step and the detail of the press-molding dies 50 and 60 used for press molding are as follows.

-Implementation conditions for the first step-
The temperature of the press molding surface 52A just before the first step is 500 ° C., the temperature of the press molding surface 62A just before the first step is 500 ° C., and the in-plane temperature difference of the press molding surface 52A just before the first step is performed. Was set to 50 ° C., and the in-plane temperature difference of the press-molded surface 62A immediately before the first step was set to 50 ° C. The press molds 50 and 60 were driven such that the press molding surface 52A and the press molding surface 62A were in contact with the molten glass lump 24 at the same time. The press molding time was 0.07 seconds. Note that the temperature of the press molding surfaces 52A and 62A was monitored by a thermocouple disposed at a depth of 30 mm from the press molding surfaces 52A and 62A. This thermocouple is one at the center of the press molding surfaces 52A and 62A, and one at a position of a radius of 1 mm from the center and at 0 °, 90 °, 180 ° and 270 ° in the circumferential direction. Has been placed.

-Implementation conditions for the second step-
The temperature (extraction temperature) of the sheet glass 26 at the end of the second step is set to be 495 ° C., and the press pressure of the press mold main bodies 52 and 62 during the second step is always 0.5 MPa. Set to maintain. The temperature of the sheet glass 26 is a value obtained on the assumption that the temperature is measured by a thermocouple arranged at the center of the press molding surfaces 52A and 62A.

-Press mold-
The press mold 50 was made of cast iron and used as an integral type in which a press mold main body 52 and a guide member 54 were integrally configured. Also, the press mold 50 was an integral type similar to the press mold 60. The press molding surfaces 52A and 62A are completely flat surfaces. Further, the used press molds 50 and 60 have flow paths for flowing cooling water inside the press mold main bodies 52 and 62 so that the temperature of the press mold surfaces 52A and 62A and the in-plane temperature distribution can be controlled. While being provided, a heater is disposed on the outer peripheral side of the press molds 50 and 60.

(Example A2)
A glass blank was produced in the same manner as in Example A1, except that the extraction temperature was set to 490 ° C.

(Example A3)
A glass blank was produced in the same manner as in Example A1, except that the extraction temperature was set to 505 ° C.

(Example A4)
A glass blank was produced in the same manner as in Example A3, except that the pressing pressure during the second step was reduced with time. Note that the press pressure was controlled to be 50% when the temperature of the sheet glass 26 reached the sag temperature of -25 ° C., with the reference immediately after the start of the second step (100%).

(Example A5)
A glass blank was produced in the same manner as in Example A3, except that the pressing pressure during the second step was reduced with time. Note that the press pressure was controlled to be 50% when the temperature of the sheet glass 26 reached the yield point + 25 ° C., with the reference immediately after the start of the second step (100%).

(Example A6)
A glass blank was produced in the same manner as in Example A3, except that the pressing pressure during the second step was reduced with time. Note that the press pressure was controlled to be 50% when the temperature of the sheet glass 26 reached the yield point of −40 ° C., with the reference immediately after the start of the second step (100%).

(Example A7)
A glass blank was produced in the same manner as in Example A3, except that the pressing pressure during the second step was reduced with time. Note that the pressing pressure was controlled to be 50% when the temperature of the sheet glass 26 reached the yield point + 40 ° C., with the reference immediately after the start of the second step (100%).

(Example A8)
A glass blank was produced in the same manner as in Example A3, except that the pressing pressure during the second step was reduced with time. The press pressure was controlled to be 50% when the temperature of the sheet glass 26 reached the yield point, with the reference immediately after the start of the second step (100%).

(Comparative Example A1)
A glass blank was basically produced under the same conditions as in Example A1, except that the press mold 200 shown in FIG. 12 was used as the press mold. However, the press pressure was applied to the entire press mold 200 during the second step.

  The press mold 200 shown in FIG. 12 is made of cast iron and has a structure in which the press mold body 52 and the guide member 54 constituting the press mold 50S shown in FIG. 10 are integrated. The press mold 200 has a cylindrical shape, and one end surface is a press molding surface 200A. Further, a ring-shaped convex portion 202 having a function similar to that of the guide member 54 is provided along the outer edge portion of the press molding surface. Further, a rod-like member 204 is attached to the surface opposite to the press-molding surface 200A, and a drive device (not shown) is connected to the other end of the rod-like member 104. The rod-shaped member 204 is attached so as to be coaxial with the axial direction X of the press mold 200. Further, the smoothness and flatness of the press molding surface 200A of the press mold 200, and the dimensions of each part of the press molding surface 200A and the convex portion 202 are substantially the same as the molding die 50S shown in FIG. The same as above.

(Comparative Example A2)
A glass blank was basically produced under the same conditions as in Example A1, except that the press mold 210 shown in FIG. 12 was used as the press mold. However, the first step ends when the thickness of the plate-like glass 26 becomes approximately the same as the thickness of the glass blank to be produced, and thereafter, the second step is carried out by reducing the press pressure. did. During the second step, the press pressure was applied to the entire press mold 210.

  The press mold 210 shown in FIG. 12 is made of cast iron and has a configuration corresponding to the press mold body 52 that constitutes the press mold 50S shown in FIG. The press mold 210 has a cylindrical shape, and one end surface is a press molding surface 210A. Further, a rod-like member 212 is attached to the surface opposite to the press-molding surface 210A, and a drive device (not shown) is connected to the other end of the rod-like member 212. The rod-shaped member 212 is attached so as to be coaxial with the axial direction X of the press mold 210.

(Evaluation)
About the glass blank produced in each Example and comparative example, flatness, a plate | board thickness deviation, and the crack were evaluated. The results are shown in Table 1. Note that the temperatures of the two press-molded surfaces during the first step and the second step in the examples and comparative examples both showed substantially the same temperature, and were at most 505 ° C. or lower.

  In addition, the evaluation method and evaluation criteria of flatness, sheet thickness deviation, and crack shown in Table 1 are as follows.

-Flatness-
The flatness was measured by using a three-dimensional shape measuring device (manufactured by COMS Corporation, high-precision three-dimensional shape measuring system, MAP-3D), and an average value of the flatness of ten samples was obtained.

−Thickness deviation−
The thickness deviation is measured with a micrometer at the center point of the produced glass blank and the positions at a radius of 30 mm and at 0 °, 90 °, 180 ° and 270 ° in the circumferential direction. Five standard deviations were determined. And the average value of the standard deviation of 10 samples was calculated | required.

-Cracking-
When 1000 glass blanks were produced continuously, the obtained glass blanks were counted, and the occurrence rate of cracks was determined. The evaluation criteria for the evaluation results shown in Table 1 and Table 2 are as follows.
A: The occurrence rate of cracks is 0%
B: Crack generation rate exceeds 0% and 1% or less C: Crack generation rate exceeds 1% and 2% or less D: Crack generation rate is 3% or more

<< Preparation of Magnetic Recording Medium Glass Substrate and Magnetic Recording Medium >>
<Example B1>
The glass blank produced in Example A1 was annealed to reduce and remove strain. Next, scribing was performed on the outer peripheral portion and the central hole portion of the magnetic recording medium glass substrate. By such processing, two concentric grooves were formed on the outer side and the outer side. Subsequently, the scribe-processed part was heated partially, the crack was generated along the scribe-processed groove | channel by the difference in thermal expansion of glass, and the outer part and inner part of the outer concentric circle were removed. Thereby, a disc-shaped glass having a perfect circle shape was obtained.

  Next, the shape of the disk-shaped glass was processed by chamfering or the like, and end face polishing was performed. Next, after subjecting the main surface of the disk-shaped glass to first polishing, the glass was immersed in a chemical strengthening solution and chemically strengthened. After chemical strengthening, the glass that was sufficiently washed was subjected to the second polishing. After the second polishing step, the disk-shaped glass was washed again to produce a magnetic recording medium glass substrate. The obtained magnetic recording medium glass substrate had an outer diameter of 65 mm, a center hole diameter of 20 mm, a thickness of 0.8 mm, and a main surface roughness of 0.2 nm or less.

  Note that in the production of the magnetic recording medium glass substrate, as described above, a process performed mainly for improving flatness such as a lapping process was omitted. However, the flatness of the glass blank used for processing was 4 μm, the flatness of the produced magnetic recording medium glass substrate was 4 μm, and there was almost no difference between the flatness of the two. The flatness of the magnetic recording medium glass substrate was measured in the same manner as the flatness of the glass blank.

  Next, using the produced magnetic recording medium glass substrate, an adhesion layer, an underlayer, a magnetic layer, a protective layer, and a lubricating layer are formed in this order on the main surface of the magnetic recording medium glass substrate. Obtained. First, an adhesion layer, a base layer, and a magnetic layer were sequentially formed in an Ar atmosphere by a DC magnetron sputtering method using a vacuum-deposited film forming apparatus. At this time, the adhesion layer was formed using a CrTi target so as to be an amorphous CrTi layer having a thickness of 20 nm. Subsequently, a 10 nm thick layer made of amorphous CrRu was formed as a base layer by a DC magnetron sputtering method in an Ar atmosphere using a single wafer / stationary facing film forming apparatus. The magnetic layer was formed at a film forming temperature of 400 ° C. using an FePt or CoPt target so as to be an amorphous FePt or CoPt layer having a thickness of 200 nm. The magnetic recording medium after film formation up to the magnetic layer was transferred from the film forming apparatus to a heating furnace and annealed at a temperature of 650 to 700 ° C.

  Subsequently, a protective layer made of hydrogenated carbon was formed by a CVD method using ethylene as a material gas. Thereafter, a lubricating layer using PFPE (perfluoropolyether) was formed by a dip coating method. The thickness of the lubricating layer was 1 nm. A magnetic recording medium was obtained by the above manufacturing process.

  The flatness of the obtained magnetic recording medium was 4 μm, which was almost the same as the flatness of the magnetic recording medium glass substrate used for the production of the magnetic recording medium. The flatness of the magnetic recording medium was measured in the same manner as the flatness of the glass blank.

<Comparative Example B1>
A magnetic recording medium glass substrate was produced using the glass blank produced in Comparative Example A1. In the production of the magnetic recording medium glass substrate, magnetic recording was performed in the same manner as in Example B1, except that the lapping step was further performed by setting the grinding allowance to 50 μm after the end face polishing and before the first polishing. A medium glass substrate was produced. The obtained magnetic recording medium glass substrate had an outer diameter of 65 mm, a center hole diameter of 20 mm, a thickness of 0.8 mm, and a main surface roughness of 0.2 nm or less. The flatness of the glass blank used for processing was 15 μm, the flatness of the produced magnetic recording medium glass substrate was 4 μm, and it was confirmed that the flatness was greatly improved.

  Next, a magnetic recording medium glass substrate was produced in the same manner as in Example B1 using the obtained magnetic recording medium glass substrate. The flatness of the obtained magnetic recording medium was 4 μm, which was almost the same as the flatness of the magnetic recording medium glass substrate used for the production of the magnetic recording medium.

<Comparative Example B2>
A magnetic recording medium glass substrate and a magnetic recording medium were produced in the same manner as in Comparative Example B1 except that the lapping step was omitted. The flatness of the obtained magnetic recording medium glass substrate and magnetic recording medium was substantially the same as the flatness of the glass blank used for processing.

DESCRIPTION OF SYMBOLS 10 Molten glass outflow pipe 12 Glass outflow port 20 Molten glass flow 22 Tip part 24 Molten glass lump 26 Sheet glass 30 Lower blade (shear blade)
32 Main body portion 34 Blade portion 34U Upper surface 34B (blade portion) Lower surface 40 (blade portion) Lower surface 40 Upper blade (shear blade)
42 Main body portion 44 Blade portion 44U Upper surface 44B (blade portion) lower surface 50 (blade portion) lower surface 50 First press mold 50S Press mold 52 Press mold body 52A Press mold surface 52B Extruded surface 54 Guide member 54A Guide surface 54B Extruded surface 56 First extruded member 56A Extruded surface 56B Surface 56H opposite to the extruded surface 56A Through hole 58 Second extruded member 60 Second press mold 62 Press mold body 62A Press mold surface 64 Guide member 64A Guide surface 70 Support member 100 Press mold

Claims (22)

At least through a press molding step of press-molding the molten glass lump that is falling with the first press mold and the second press mold that are arranged to face each other in the direction intersecting the dropping direction of the molten glass lump. Manufacturing glass blanks for magnetic recording medium glass substrates,
At least the first press mold is
A press mold body having a press molding surface;
At the time of press molding, the first press is brought into contact with a part of the press mold disposed opposite to the press molding surface when being extruded to the side of the press mold disposed opposite to the press molding surface. A guide member having at least a function of maintaining a substantially constant distance between the press mold surfaces of the mold and the second press mold,
The press molding process is
The molten glass is made by bringing the first press mold and the second press mold close to each other until the guide member of the first press mold and the second press mold come into contact with each other. A first step of forming the mass into sheet glass;
In a state where the guide member of the first press mold and the second press mold are in contact with each other, the press mold body of the first press mold, the second press mold, A second step of further pressing the plate glass by
The manufacturing method of the glass blank for magnetic recording medium glass substrates characterized by including these. In the manufacturing method of the glass blank for magnetic recording medium glass substrates of Claim 1,
Each of the first press mold and the second press mold is
A press mold body having a press molding surface;
At the time of press molding, the first press is brought into contact with a part of the press mold disposed opposite to the press molding surface when being extruded to the side of the press mold disposed opposite to the press molding surface. A guide member having at least a function of maintaining a substantially constant distance between the press mold surfaces of the mold and the second press mold,
The first step is
By bringing the first press mold and the second press mold close to each other until the guide member of the first press mold and the guide member of the second press mold are in contact with each other. Implemented,
Said second step comprises
In a state where the guide member of the first press mold and the guide member of the second press mold are in contact with each other, the press mold body of the first press mold and the second press mold A method for producing a glass blank for a magnetic recording medium glass substrate, which is carried out by further pressing the plate glass with a press mold body of a mold. In the manufacturing method of the glass blank for magnetic recording medium glass substrates of Claim 1 or 2,
Including a molten glass lump forming step of forming the molten glass lump by suspending the molten glass from the glass outlet and cutting the tip of the molten glass flow continuously flowing out downward in the vertical direction. A method for producing a glass blank for a magnetic recording medium glass substrate. In the manufacturing method of the glass blank for magnetic recording medium glass substrates of Claim 3,
The method for producing a glass blank for a magnetic recording medium glass substrate, wherein the molten glass has a viscosity of 500 dPa · s to 1050 dPa · s. In the manufacturing method of the glass blank for magnetic recording medium glass substrates as described in any one of Claims 1-4,
A glass blank for a magnetic recording medium glass substrate, wherein the first press mold and the second press mold are disposed to face each other in a direction orthogonal to a falling direction of the molten glass lump. Production method. In the manufacturing method of the glass blank for magnetic recording medium glass substrates as described in any one of Claims 1-5,
A method for producing a glass blank for a magnetic recording medium glass substrate, wherein the duration of the second step is controlled so that the flatness of the glass blank for a magnetic recording medium glass substrate is 10 μm or less. In the manufacturing method of the glass blank for magnetic recording medium glass substrates as described in any one of Claims 1-6,
The duration of the second step is equal to or lower than the temperature at which the temperature of the sheet glass at the end of the second step is at least 10 ° C. added to the strain point of the glass material constituting the sheet glass. It selects so that it may become. The manufacturing method of the glass blank for magnetic recording medium glass substrates characterized by the above-mentioned. In the manufacturing method of the glass blank for magnetic recording medium glass substrates as described in any one of Claims 1-7,
The absolute value of the difference between the temperature of the press molding surface of the first press mold and the temperature of the press molding surface of the second press mold immediately before performing the first step is 0 ° C. to The manufacturing method of the glass blank for magnetic recording medium glass substrates characterized by being in the range of 10 degreeC. In the manufacturing method of the glass blank for magnetic recording medium glass substrates as described in any one of Claims 1-8,
The absolute value of the in-plane temperature difference between the press molding surfaces of the first press mold and the second press mold immediately before performing the first step is in the range of 0 ° C to 100 ° C. A method for producing a glass blank for a magnetic recording medium glass substrate. In the manufacturing method of the glass blank for magnetic recording medium glass substrates as described in any one of Claims 1-9,
The temperature of the press molding surface of the first press mold and the temperature of the press molding surface of the second press mold at least during the press molding step are substantially the same, and ,
Press-molding the molten glass mass after bringing the press-molding surface of the first press-molding die and the press-molding surface of the second press mold into contact with the molten glass mass almost simultaneously. A method for producing a glass blank for a magnetic recording medium glass substrate. In the manufacturing method of the glass blank for magnetic recording-medium glass substrates as described in any one of Claims 1-10, The temperature of the said plate glass is 10 degreeC at the strain point of the glass material which comprises the said plate glass at least. The method for producing a glass blank for a magnetic recording medium glass substrate, wherein the second step is continued until the temperature is lower than the applied temperature. In the manufacturing method of the glass blank for magnetic recording medium glass substrates as described in any one of Claims 1-11,
A method for producing a glass blank for a magnetic recording medium glass substrate, wherein the pressing pressure in the second step is decreased over time. In the manufacturing method of the glass blank for magnetic recording medium glass substrates of Claim 12,
The temperature of the plate glass sandwiched between the first press mold and the second press mold is the yield point of the glass material constituting the plate glass ± 30 ° C. A method for producing a glass blank for a glass substrate for a magnetic recording medium, wherein the glass blank is reduced when the temperature falls within the range. In the manufacturing method of the glass blank for magnetic recording medium glass substrates as described in any one of Claims 1-13,
The flatness of the glass blank for a magnetic recording medium glass substrate is 10 μm or less, The method for producing a glass blank for a magnetic recording medium glass substrate. In the manufacturing method of the glass blank for magnetic recording medium glass substrates as described in any one of Claims 1-14,
The flatness of the glass blank for a magnetic recording medium glass substrate is 4 μm or less, The method for producing a glass blank for a magnetic recording medium glass substrate. In the manufacturing method of the glass blank as described in any one of Claims 1-15,
A glass blank for a magnetic recording medium glass substrate, wherein at least a region of the press forming surface of the first press mold and the second press mold that contacts the plate glass is a substantially flat surface. Manufacturing method. In the manufacturing method of the glass blank as described in any one of Claims 1-16,
Each of the first press mold and the second press mold is
A first extruding member that extrudes the press mold main body and the guide member at the same time in a direction perpendicular to the press molding surface and on the press mold side disposed opposite to the press molding surface;
After the first extruding member makes contact with the guide member and a part of the press mold placed opposite to the press molding surface, the press mold main body is in a direction perpendicular to the press molding surface. And a second extruding member for extruding to a press mold side disposed opposite to the press molding surface, and a method for producing a glass blank for a magnetic recording medium glass substrate. At least through a press molding step of press-molding the molten glass lump that is falling with the first press mold and the second press mold that are arranged to face each other in the direction intersecting the dropping direction of the molten glass lump. After manufacturing a glass blank for a magnetic recording medium glass substrate,
Producing a magnetic recording medium glass substrate through at least a polishing step for polishing the main surface of the glass blank for magnetic recording medium,
At least the first press mold is
A press mold body having a press molding surface;
At the time of press molding, the first press is brought into contact with a part of the press mold disposed opposite to the press molding surface when being extruded to the side of the press mold disposed opposite to the press molding surface. A guide member having at least a function of maintaining a substantially constant distance between the press mold surfaces of the mold and the second press mold,
The press molding process is
The molten glass is made by bringing the first press mold and the second press mold close to each other until the guide member of the first press mold and the second press mold come into contact with each other. A first step of forming the mass into sheet glass;
In a state where the guide member of the first press mold and the second press mold are in contact with each other, the press mold body of the first press mold, the second press mold, A second step of further pressing the plate glass by
The manufacturing method of the magnetic-recording-medium glass substrate characterized by including these. In the manufacturing method of the magnetic-recording-medium glass substrate of Claim 18,
A method for producing a magnetic recording medium glass substrate, wherein the flatness of the glass blank for the magnetic recording medium glass substrate and the flatness of the magnetic recording medium glass substrate are substantially the same. At least through a press molding step of press-molding the molten glass lump that is falling with the first press mold and the second press mold that are arranged to face each other in the direction intersecting the dropping direction of the molten glass lump. After manufacturing a glass blank for a magnetic recording medium glass substrate,
Producing a magnetic recording medium glass substrate through at least a polishing step of polishing the main surface of the glass blank for magnetic recording medium,
At least through a magnetic recording layer forming step of forming a magnetic recording layer on the magnetic recording medium glass substrate, a magnetic recording medium is produced,
At least the first press mold is
A press mold body having a press molding surface;
At the time of press molding, the first press is brought into contact with a part of the press mold disposed opposite to the press molding surface when being extruded to the side of the press mold disposed opposite to the press molding surface. A guide member having at least a function of maintaining a substantially constant distance between the press mold surfaces of the mold and the second press mold,
The press molding process is
The molten glass is made by bringing the first press mold and the second press mold close to each other until the guide member of the first press mold and the second press mold come into contact with each other. A first step of forming the mass into sheet glass;
In a state where the guide member of the first press mold and the second press mold are in contact with each other, the press mold body of the first press mold, the second press mold, A second step of further pressing the plate glass by
A method for manufacturing a magnetic recording medium, comprising: The method of manufacturing a magnetic recording medium according to claim 20,
A method for producing a magnetic recording medium, wherein the flatness of the glass blank for the magnetic recording medium glass substrate and the flatness of the magnetic recording medium glass substrate are substantially the same. A molten glass outflow pipe provided with an outlet for dropping the molten glass flow downward in the vertical direction;
It is a direction substantially orthogonal to the direction in which the molten glass flow flowing out from the molten glass outflow pipe hangs down, and is opposed to both sides of the direction in which the molten glass flow hangs down, and penetrates from both sides of the molten glass flow. A pair of shear blades that cut the tip of the molten glass flow to form a molten glass lump,
A direction that is substantially orthogonal to the direction in which the molten glass lump falling downward in the vertical direction is opposed to both sides of the direction in which the molten glass lump falls, and sandwiches the molten glass lump from both sides. A first press mold and a second press mold for press-molding the molten glass mass into a sheet glass by,
At least the first press mold is
A press mold body having a press molding surface;
At the time of press molding, the first press is brought into contact with a part of the press mold disposed opposite to the press molding surface when being extruded to the side of the press mold disposed opposite to the press molding surface. A guide member having at least a function of maintaining a substantially constant distance between the press mold surfaces of the mold and the second press mold; and
A first extruding member that extrudes the press mold main body and the guide member at the same time in a direction orthogonal to the press molding surface and on the press mold side disposed opposite to the press molding surface;
After the guide member and a part of the press mold disposed opposite to the press molding surface are brought into contact with each other by the first extruding member, the press mold main body is in a direction perpendicular to the press molding surface. And a second extruding member for extruding to the press mold side disposed opposite to the press molding surface,
An apparatus for producing a glass blank for a magnetic recording medium glass substrate, comprising:

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