JP2008110897A - Preform for precision press molding, and method of manufacturing optical element from the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a preform capable of remarkably reducing defective products when in precision press molding, and conducting efficiently a process of manufacturing an optical element. <P>SOLUTION: The preform for precision press molding manufactured by molding a molten glass has a strain of not greater than 10 nm/cm on the basis of JOGIS14-1975 (method of measuring a strain of an optical glass). The preform is characterized by applying heat treatment in order to reducing the strain. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a method of manufacturing an optical element by creating a preform from optical glass and precision press-molding the preform. Furthermore, the present invention is a method for producing a precision press-molding preform by melting and molding optical glass, the manufacturing method including a heat treatment step for removing strain, and the manufacturing method. The present invention relates to a method of manufacturing an optical element using a preform. Further, the present invention relates to a preform having a small distortion used in the manufacturing method.

  In recent years, in the field of optical devices such as digital cameras and projectors, downsizing and weight reduction have been demanded, and accordingly, downsizing of optical elements and reduction of the number of lenses used have become issues.

In general, the lenses constituting the optical system generally include a spherical lens and an aspheric lens. Many spherical lenses are manufactured by cold working (grinding / polishing, etc.) a glass material, or by cold working (grinding / polishing, etc.) a glass molded product obtained by reheat press molding. . On the other hand, an aspherical lens is formed by press-molding a heat-softened spherical, elliptical, or flat preform with a mold having a high-precision molding surface, and using the shape of the high-precision molding surface of the mold as a preform material. The mainstream is a method obtained by transfer, that is, manufactured by precision press molding. However, except when making a specially shaped lens such as an aspherical lens, most of the optical elements were manufactured by the conventional method because it is cheaper to finish by cold working.

There are mainly the following methods for producing optical elements that are finished by conventional cold working.
(1) By melting molten glass, flowing out from one end, nozzle or the like onto a mold, forming a plate-like or rod-like glass lump, and then performing cold processing such as cutting, grinding, polishing, etc. A method of forming into a desired shape and then precision annealing, or a method of finely annealing and then polishing the glass lump after cutting, grinding.
(2) As in (1), a plate-shaped or rod-shaped glass molded body is formed, the glass molded body is cut and subjected to cold processing such as grinding, and then the glass block is reheat press-molded. A method in which a reheat press-molded product is molded into a desired shape by polishing or the like and then precision annealed, or a glass lump is cut, ground, precision annealed and then polished.

Although it is an old method, there are mainly the following methods as a method of manufacturing an optical element that is molded into a desired shape by hot working.
(3) A method in which a molten glass flow flowing out from a nozzle or the like is separated using a shear or surface tension to form a glass lump, the glass lump is hot-formed, and then precision annealed, or thereafter polished.

In any conventional manufacturing method, it is necessary to finally perform precision annealing. Here, the precision annealing is a process of applying a precisely controlled thermal history for the purpose of removing distortion of the optical element, adjusting the refractive index, and homogenizing the inside of the glass, and usually takes a long time (eg, several days to several tens of days). ), Which was a cause of high costs.

Such precision annealing is known from, for example, JP-A-5-306133 (hereinafter referred to as Patent Document 4) and JP-A-9-268022 (hereinafter referred to as Patent Document 5). It was a process requiring a very long heat treatment. Note that these are all processes performed on the final optical element.

  In recent years, the demands for lenses on these conventional methods have changed, and the demand for specially shaped lenses such as aspherical lenses has increased. Accordingly, many optical glass preforms for use in precision press molding have been manufactured.

Next, an optical glass preform for use in precision press molding will be described.

When a preform is used for precision press molding, the temperature of the precision press process itself is usually precisely controlled, so the thermal history of the preform before molding is eliminated and unified with a new thermal history. The That is, even if the preform has a refractive index variation that depends on the thermal history at the time of preparation, the thermal history of the preform is eventually eliminated by the precision pressing process, resulting from a new thermal history at the time of precision pressing. Will be unified to the refractive index. Thereby, it can be said that the refractive index of the preform itself is no longer required to have a high degree of accuracy even after the precision annealing.

Various techniques are known as a method for producing a precision press-molding preform.

First, the above-described conventional optical element manufacturing methods (1) to (3) are directly applied to preform manufacturing. When this method is used, it is often omitted because the necessity of the final precision annealing step is poor as described above.

Furthermore, a molding method that does not use such cold working is also known. In particular, in recent years, molten glass flowing out from a nozzle or the like has an appropriate size and shape by being dropped as glass droplets using shear or surface tension, for example, onto a porous mold that ejects gas, and then floated. The technology to adjust to glass gob is used. However, in the former case, traces of cutting by the shear may remain on the glass gob, so in recent years the latter is often used.

A number of techniques are known as methods for producing such precision press-molding preforms from molten glass. JP-A-2003-20248 (hereinafter referred to as Patent Document 1), JP-A-11-116252 (hereinafter referred to as Patent Document 2), and JP-A-11-171565 (hereinafter referred to as Patent Document 3) A method is described in which molten glass is supplied onto a forming mold to be floated and formed. All of these cool the glass in a state of quenching or near quenching, and many are shipped without precision annealing.

JP 2003-20248 A Japanese Patent Laid-Open No. 11-116252 JP 11-171565 A JP-A-5-306133 Japanese Patent Laid-Open No. 9-268022

However, a preform without precision annealing may cause defects such as cracks and chipping due to its own strain in the subsequent precision pressing process, that is, a series of molding processes that are heated, pressed, held, and cooled. There were so many cases that it was a cause of deterioration in yield and work efficiency. In addition, the occurrence of these defective products may cause damage to expensive press mold films and the like, and it has been an urgent task in the industry to solve these problems. On the other hand, precision annealing, which is usually performed on optical glass, especially sheet glass, has the disadvantages described above, which can lead to a sharp increase in cost. It is unrealistic to apply.

The present inventor has paid attention to the residual strain generated in the rapid cooling process at the time of forming the preform as a major cause of the occurrence of the above-described defect.

That is, as a result of various studies, the inventor added a simple thermal history only to reduce the remaining strain to a predetermined range in the preform created through the rapid cooling step, thereby reducing the residual strain. It has been found that the use of preforms that have been reduced can significantly reduce the occurrence of defects during precision press molding and efficiently perform the optical element manufacturing process.

The first configuration of the present invention is a precision press-molding preform manufactured by molding molten glass, which is 10 nm / cm or less under the conditions of JOGIS14-1975 (a method for measuring strain of optical glass). The preform having distortion.

The second configuration of the present invention is the precision press-molding preform, wherein a heat treatment for reducing strain is performed.

According to a third configuration of the present invention, a glass lump is formed and obtained by separating a molten glass stream, and the glass lump is formed by flotation molding on a mold. Preform for precision press molding.

According to a fourth configuration of the present invention, the first and second configurations are manufactured by forming a molten glass stream into a plate-like or rod-like glass molded body, and hot and / or cold working the glass molded body. Preform for precision press molding.

In the fifth configuration of the present invention, the heat treatment (1) raises the temperature of the glass molded body in a room temperature state to a temperature range from the strain point to the yield point at a rate of 50 to 200 ° C./hr,
(2) Hold at the elevated temperature for 6 hours or less,
(3) The temperature is lowered from the held temperature to a temperature lower by 80 to 200 ° C. at a speed of −10 to −40 ° C./hr,
(4) After that, let it cool naturally.
It is a precision press-molding preform having the above-described configurations 1 to 4 including a process

The sixth configuration of the present invention is characterized in that the refractive index of the preform has a refractive index obtained by subtracting a refractive index change generated during precision press molding from a refractive index required for an optical element. It is the preform for precision press molding of the structures 1-5.

The seventh configuration of the present invention is a method of manufacturing an optical element by precision press-molding a preform made of optical glass, and the distortion is reduced by heat-treating the preform before precision press-molding. It is the said manufacturing method including the process to make.

In the eighth configuration of the present invention, a preform base material made of optical glass is heat-treated to reduce distortion, and the preform base material after the heat treatment is manufactured by cold or hot processing, In this method, an optical element is manufactured by precision press-molding the preform.

According to a ninth configuration of the present invention, the preform has a strain of 10 nm / cm or less under the conditions of JOGIS14-1975 (method for measuring strain of optical glass). Is the method.

In the tenth configuration of the present invention, the heat treatment (1) raises the temperature of the glass molded body in a room temperature state to a temperature range from the strain point to the yield point at a rate of 50 to 200 ° C./hr,
(2) Hold at the elevated temperature for 6 hours or less,
(3) The temperature is lowered from the held temperature to a temperature lower by 80 to 200 ° C. at a speed of −10 to −40 ° C./hr,
(4) After that, let it cool naturally.
It is a manufacturing method of the said structures 7-9 including a process.

The manufacturing process of the preform of the present invention will be described.

FIG. 1 shows an embodiment of the preform manufacturing process of the present invention, and shows a process of forming a preform by forming a molten glass into a sheet glass and then cold working. In general, optical glass melts a glass raw material in a crucible (S11), and the molten glass passes through a nozzle and continuously flows out to one end of a mold. The outflowed glass is drawn out from the other end of the mold, and is formed as a sheet glass (S12). A glass lump is created by performing cold working 1 for cutting / grinding the formed sheet glass (S13), and in some cases, reheat pressing (S14) for grinding, polishing, etc. By performing cold working 2 (S15), a precision press-molding preform is completed (S16).

FIG. 2 illustrates a process (S11 to 12) apparatus for forming molten glass into a sheet glass.

In the step (S12) of flowing molten glass onto a mold and molding (S12), the molten glass is usually discharged from the melting crucible through a nozzle and rapidly cooled by flowing down onto the mold, resulting in a considerable amount of distortion. I think that it is included. Here, too much strain causes defects such as cracking even in the subsequent cold working, so when forming or passing the sheet glass immediately after forming through a slow cooling furnace, a very simple predetermined It is common to add a thermal history (hereinafter referred to as rare annealing).

The glass lump produced by cold working (S13) is reheat-pressed when the precision press preform of the desired shape is finally made (S16), and the gob at the previous stage is made into a more final shape. It is because it is necessary to make it close. Since the heat history after the reheat press is hardly managed precisely, the reheat pressed glass molded product (hereinafter referred to as a reheat press product) often has a considerable amount of distortion.

As mentioned above, when a precision press-molding preform is manufactured by a conventional manufacturing method, the glass does not have a precisely controlled thermal history, and the refractive index varies, and the reheat press product. In many cases, the strain possessed by had remained.

In the present invention, a simple heat treatment for strain relief (hereinafter referred to as strain relief heat treatment) is performed immediately after the reheat press (S14) step (S14 ′) and / or immediately after the cold working 2 (S15) (S15 ′). It is preferable to carry out (flow by a dashed arrow). This is because even if heat treatment without strain is performed before the reheat press, strain is generated again in the reheat press.
On the other hand, when the reheat press is not performed, a heat treatment is performed after the cold working 1 (S13 ′), and then a zero winding process 2 is performed (S15).

In the case of performing strain relief heat treatment immediately after the reheat press (S14) step (S14 ′), the residual heat at the time of reheat press is used, and after the reheat press, it is not rapidly cooled but kept at a predetermined temperature, and the cooling rate is slowed down. Therefore, the generation of distortion may be suppressed. According to this method, the step of reheating the glass molded product can be omitted.

FIG. 3 shows another embodiment of the preform manufacturing process according to the present invention, and shows a process of directly forming a preform by separating a molten glass lump from molten glass and performing float forming. The optical glass melts a glass raw material in a crucible (S21), and the molten glass passes through a nozzle and is separated into a glass lump by cutting with a shear at the nozzle tip or by the action of surface tension (S22). The separated glass lump is received by a receiving mold and molded there (S23). FIG. 4 illustrates an apparatus used for the step of float forming molten glass (S21 to 23).

Here, it is preferable that the receiving mold is provided with one or more air ejection holes, or the receiving mold itself is made of a porous material and can eject gas. The glass block is float-molded on the receiving mold to complete a spherical or elliptical preform (S24).

In the present invention (flow by broken-line arrows), it is particularly preferable to perform strain relief heat treatment immediately after float forming (S23) or during float forming (S23 '). When it is performed during molding, the glass lump on the receiving mold may be heated with a heater, and adjustments such as raising the temperature of the gas to be ejected can be performed.

The degree of strain relief naturally depends on the heating rate, holding time, pressing pressure, and cooling rate in precision press molding in the subsequent process. However, when using precision press equipment and conditions normally used in the field of optical glass, the amount of residual strain in the precision press molding preform is defined in “JOGIS14-1975 (Measurement Method of Optical Glass Strain)”. According to the measurement method, it is preferably 10 nm / cm or less, more preferably 9 nm / cm or less, and most preferably 8 nm / cm or less. Needless to say, the lower the strain, the better. However, excessive precision annealing is not required as long as it does not cause defects in precision press molding.

The conditions for the strain relief heat treatment can be appropriately selected as long as the strain is removed to such an extent that no defects are caused during precision press molding. However, the following conditions are generally preferred for precision press-molding glass materials that are usually used in the optical glass industry.

In the present specification, for convenience of explanation, the original material for producing a precision press-molding preform (that is, a glass lump cold-worked from a plate material or the like, a reheat press product of the glass lump, or a float-molded product) Collectively called “preform base material”. In other words, the preform may be created by polishing the preform base material, or it may not be necessary.

The strain relief heat treatment for the preform base material is often performed by a process in which the preform base material is heated, held at a constant temperature, and cooled, but originally when the glass molded product is at a high temperature (for example, in the reheat press process) In the case of performing strain relief using residual heat), one or both of the heating step and the holding step may be omitted.

When the preform base material is heated, it is preferably heated to a temperature range from the strain point to the yield point. If this temperature is too high, the preform base material is likely to be deformed, and it becomes difficult to obtain a preform having a desired shape. On the other hand, if the heating temperature is too low, the strain is not easily removed and the object of the present invention cannot be achieved.

The temperature rise rate of the preform base material is preferably 50 to 200 ° C./hr. If this speed is too high, defects such as cracking of the glass are likely to occur, and if it is too slow, it takes a long time to be meaningless, contrary to the gist of the present invention in which the minimum heat treatment for precision press molding is performed. It causes high cost.
That is, the rate of temperature rise of the preform base material in the present invention is preferably 50 ° C./hr, more preferably 60 ° C./hr, most preferably 70 ° C./hr as the lower limit, preferably 200 ° C./hr, more preferably The upper limit is 190 ° C./hr, most preferably 180 ° C./hr.

The holding time at the elevated temperature is preferably 6 hours or less. If this holding time is too long, the strain is reduced, but contrary to the gist of the present invention that the minimum necessary heat treatment for precision press molding is performed, it takes a long time and causes a high cost. That is, the holding time of the preform base material at a predetermined temperature in the present invention is preferably 6 hours, more preferably 5.5 hours, and most preferably 5 hours. Although there is no particular lower limit, it is preferably maintained for 0.5 hour or longer, more preferably 1.0 hour or longer, and most preferably 1.5 hour or longer.

When cooling the preform base material held at the predetermined temperature, the preform base material is not rapidly cooled to the predetermined temperature so as not to cause new distortions or cracks in the preform base material. It is preferable to lower the temperature at a constant rate. Specifically, the temperature is lowered at a constant rate, preferably to a temperature lower by 80 to 200 ° C., more preferably to a temperature lower by 90 to 180 ° C., and most preferably to a temperature lower by 80 to 150 ° C. than the held temperature. Let

In this case, the rate of temperature reduction is preferably −10 ° C./hr, more preferably −15 ° C./hr, most preferably −20 ° C./hr, preferably −40 ° C./hr, more preferably −35. The upper limit is set to ° C / hr, most preferably -30 ° C / hr. If this speed is too high, new distortion will occur and the preform will be unsuitable for precision presses. If this speed is too slow, the minimum necessary heat treatment for precision press molding will be performed. It takes a long time to make sense, which leads to high costs.

After cooling to the predetermined temperature, natural cooling may be performed. Of course, it is also possible to cool to room temperature at a constant rate, but it goes against the spirit of the present invention and incurs unnecessary costs.

The preform raw material becomes a precision press-molding preform as it is or after a further cold working process after the heat treatment for removing strain. The refractive index of this precision press-molding preform may not be the same as the target refractive index of the optical element that is the final product. Preferably, it has a refractive index obtained by subtracting the refractive index change caused by the thermal history applied to the glass to be molded in the subsequent precision press molding to the target refractive index of the optical element as the final product. That is, when the target refractive index of the optical element is n1 and the refractive index of the glass to be molded decreases by Δn in the precision press molding process, the refractive index n2 = n1 − (− Δn) of the precision press molding preform is Become. Of course, the refractive index Δn does not need to be measured for each glass, and for example, no problem occurs even if a representative value for each lot is used.

Thus, if the refractive index of the preform is determined in consideration of the change in refractive index in precision press molding in advance, the precision annealing step after precision press molding can be omitted, which is very advantageous in terms of cost. .

The glass used as the precision press-molding preform of the present invention is not particularly limited, but from the viewpoint of workability of precision press molding, the glass transition point is preferably 650 ° C. or less, more preferably 630 ° C. Hereinafter, most preferably glass of 600 ° C. or lower is used. In addition, it is particularly useful for lanthanum-based glasses containing a relatively large amount of alkali metal oxide having a large expansion.

The preform base material to which heat treatment is applied when producing the precision press-molding preform of the present invention is not limited in size, but if it is too small, the strain is originally small and there is no need for heat treatment. is there. From such a viewpoint, the preform is particularly useful for a preform base material having a diameter of preferably 10 m or more, more preferably 15 mm or more, and most preferably 20 mm or more.

When the preform material has a substantially cylindrical shape, the thickness is preferably 3 mm, more preferably 5 mm, and most preferably 8 mm.

Further, the volume of the preform base material is useful for a base material of 0.5 cm ³ or more, particularly useful for a base material of 1.0 cm ³ or more, and extremely useful for a base material of 2 cm ³ or more.

The apparatus and molding conditions for precision pressing the preform for precision press molding of the present invention are not particularly limited, and pressing can be performed using known apparatuses and conditions.

In addition, the above embodiment was illustrated using the typical manufacturing method of preform, and the manufacturing method itself is not limited. For example, the preform base material may be a glass material that is flown down from a nozzle without being floated and then naturally formed into a spherical shape in the air or in a solvent. Alternatively, a so-called abrasive ball obtained by grinding a block material into a spherical shape may be used.

From optical glass A (refractive index: 1.8, Abbe number: 40, yield point: 560 ° C., strain point: 495 ° C.) mainly composed of SiO ₂ , B ₂ O ₃ , La ₂ O ₃ , ZnO, Preform for precision press molding was created.
That is, the optical glass A is melted at 1000 to 1300 ° C. in a platinum crucible, flows out from the platinum nozzle to one end of the molding die, and the plate-like glass is drawn out from the other end of the molding die to obtain a plate shape having a thickness of 15 mm. Glass was molded. Rare annealing was performed on the sheet glass so that it would not break in the cold working of the subsequent process. Next, the plate-like glass is cut into a square of 150 mm × 115 mm, and according to the measurement method defined in “JOGIS14-1975 (Measurement Method of Optical Glass Strain)”, near the center of each side of the square and from the edge It was 28.3 nm / cm when the average which measured the distortion | strain of the place which entered 6 mm (a total of 4 places) was calculated.

This plate-like glass was further cut, ground and polished to be processed into a cylindrical shape having a diameter of 36 mm and a thickness of 10.3 mm, and this was used as a preform base material. The preform material was subjected to the following strain relief heat treatment in a hot air oven.

(1) The temperature was raised from room temperature (25 ° C.) to 510 ° C. over 5 hours (97 ° C./hr).
(2) Next, it was kept at 510 ° C. for 5 hours.
(3) Next, it was cooled to 385 ° C. over 5 hours (−25 ° C./hr).
(4) Removed from the hot stove and allowed to cool.

It was 5.5 nm / cm when the average which measured the distortion of the point (total 4 places) which entered 3 mm from the outer periphery was calculated for the distortion of the glass molded object reheat-pressed cooled to room temperature. By polishing the surface of the glass molded body, a precision press-molding preform could be obtained.

In this way, by performing the simple distortion-free heat treatment as described above on the glass molded body after reheat pressing, the distortion is greatly reduced, and a preform that can be suitably used for precision press molding can be manufactured. I understood.

Process drawing showing manufacturing process of preform of the present invention Apparatus used in the manufacturing process of the preform of the present invention Process drawing showing manufacturing process of preform of the present invention Apparatus used in the manufacturing process of the preform of the present invention

Claims (10)

A precision press-molding preform produced by molding molten glass, wherein the preform has a strain of 10 nm / cm or less under the conditions of JOGIS14-1975 (a method for measuring strain of optical glass). 2. The precision press-molding preform according to claim 1, which is subjected to a heat treatment for reducing strain. The precision press-molding preform according to claim 1 or 2, wherein a glass lump is formed and obtained by separating a molten glass stream, and the glass lump is formed by flotation molding on a mold. The precision press-molding preform according to claim 1 or 2, wherein the molten glass stream is formed into a plate-shaped or rod-shaped glass molded body, and the glass molded body is manufactured by hot and / or cold working. The heat treatment (1) raises the temperature of the glass molded body in a room temperature state to a temperature range from the strain point to the yield point at a rate of 50 to 200 ° C./hr,
(2) Hold at the elevated temperature for 6 hours or less,
(3) The temperature is lowered from the held temperature to a temperature lower by 80 to 200 ° C. at a speed of −10 to −40 ° C./hr,
(4) After that, let it cool naturally.
The precision press-molding preform according to any one of claims 1 to 4, comprising a step. 6. The precision according to claim 1, wherein the refractive index of the preform has a refractive index obtained by subtracting a refractive index change generated during precision press molding from a refractive index required for an optical element. Preform for press molding. A method for producing an optical element by precision press-molding a preform made of optical glass, comprising the step of reducing distortion by heat-treating the preform before performing precision press-molding. A preform base material made of optical glass is heat-treated to reduce distortion, the preform base material after the heat treatment is manufactured by cold or hot working, and the preform is precision press-molded. A method of manufacturing an optical element by the method. The method according to claim 7 or 8, wherein the preform has a strain of 10 nm / cm or less under the conditions of JOGIS14-1975 (method for measuring strain of optical glass). The heat treatment is (1) raising the temperature of the preform at room temperature to a temperature range from the strain point to the yield point at a rate of 50 to 200 ° C./hr,
(2) Hold at the elevated temperature for 6 hours or less,
(3) The temperature is lowered from the held temperature to a temperature lower by 80 to 200 ° C. at a speed of −10 to −40 ° C./hr,
(4) After that, let it cool naturally.
The manufacturing method in any one of Claims 7-9 including a process.

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