Mold and molding method for molding optical elements

Abstract: A mold comprises a pair of first (1) and second (2) elements each having a molding surface for defining a cavity and a rear surface (1b,2b) being opposite to the molding sur­ face and in contact with a corresponding heater block, and a ring member (3) for guiding the first and second elements. At least one rear surface has a part (5a) which is not in contact with the corresponding heater block. Also, another mold comprises a second ring member made of a material with a lower thermal conductivity than that of the first one at the outside of the first one. A molding method for optical elements is such that a glass with 10¹² poise or more is fed to a mold with a tem­ perature of a glass transition point or less, heated to a temperature corresponding to the glass viscosity 10⁸ to 10¹⁰ poise, pressed for 3 to 90 seconds, then cooled at a cooling rate 1.5 to 2.5°C/sec to 10¹⁰ to 10¹¹ poise and thereafter at a cooling rate 0.2 to 1.5°C/sec with the glass is pres­ surized, and taken out as an optical element at a tempera­ ture of a glass transition point or less.

Description: The present invention relates to a mold used mainly for press molding of a highly precise optical element and a method for molding optical elements. Recently, optical elements have tended to have a non-spherical surface with which both simplification of lens configuration and weight reduction of the lens part in an optical apparatus can be simultaneously achieved. U.S. Pat. No.4,481,023 proposes a method of manufacturing a highly precise glass product with a low cost, by which a highly precise optical glass element can be obtained by shortening the time required for a pressurization cycle. The method proposed is outlined as follows. Using a glass preform having a shape similar to its final product, a mold and the glass preform are heated separately from each other, a pressing operation is started at a temperature corresponding to the glass viscosity 10⁷Pa.s (10⁸ poise) or more and 10¹¹Pa.s (10¹² poise) or less, respectively for glass and metal molds, and the pressing operation is finished at a temperature corresponding to a glass viscosity less than 10¹²Pa.s (10¹³ poise). Then the glass is taken out. Japanese Pat. published No. 56-378/1981 discloses a method for molding a raw lens material in which the temperature of a mold is kept at a value equal to or greater than the transition point and the softening point of a molded glass, a glass having flow properties is poured into the mold to be press molded, and this condition is held for 20 seconds or more until the temperature distribution of the glass becomes uniform. However, in the prior art examples mentioned above, a glass and a mold have been separately heated and then the glass has been supplied to inside the mold to be press molded, so that in the course of press molding the temperature is very liable to be non-uniform, requiring the press to be held for a certain time or more. Since a heating section is provided on the entire peripheral portion of the mold, rapid cooling is impossible. Accordingly, this has negatively affected cycle time, with the result that a low-cost lens cannot be manufactured. As disclosed in Japanese Pat. laid open application No.61-26528/1986, a method has been known by which, after press molding, cooling is performed by using many molds and sequentially transferring them to a temperature-decline type slow cooling chambers. Also, the cooling rate after press molding is a very important parameter which affects lens performance. Japanese Pat. laid open application No. 61-53126/1986 discloses a cooling at a constant rate of 0.9°C/sec or less without pressurization. Thus, the temperature control from press molding to cooling has been known to be important in order to obtain a highly precise transferred surface. Slowing the cooling rate causes the cycle time to become long, making the supply of a low-cost optical element difficult. Also, an optical element with a substantially different wall thickness between its center portion and peripheral portion is liable to develop a temperature difference, making it difficult to obtain a desired lens performance. Further, U.S. Pat. No. 2,292,917 discloses a mold through which a thick-walled lens part is liable to cool and which uses two different materials in thermal conduction in order to make it difficult to develop a temperature difference between the center portion and the peripheral portion of an optical element. However, the mold is complex in structure, making it difficult for the mold to be located between a pair of heater blocks and transferred. Also, U.S. Pat. application serial No. 07/198,929 (filed on May 24, 1988) discloses a method by which a thin-walled lens portion is heated so as not to develop a temperature difference in the plastics material inside the cavity. However, a heating section is provided on the peripheral portion of a mold, causing the cycle time to become long. U.S. Pat. 4629489 discloses a mold for molding a material into an optical element, this mold having the features of the pre-characterizing portion of claim 1. It is an object of the present invention to provide a mold and a molding method for optical elements for making it easy to reduce a temperature difference in a glass to be molded and for quickly manufacturing a highly precise and low-cost optical element. The present invention provides a mold for molding a molding material into an optical element, comprising: first and second elements each of which includes an inner molding surface adapted to contact the molding material and an outer surface opposite said inner molding surface, said inner molding surfaces of said first and second elements defining a cavity therebetween in which the molding material is adapted to be received and molded to form the optical element; a first ring member mounted about said first and second elements and having means, comprising an inner surface thereof, for guiding movement of said first and second elements towards and away from each other; and a second ring member mounted about said first ring member, characterised in that said mold is adapted to be mounted between a pair of heater blocks, said outer surface of each of said first and second elements being adapted to contact a respective one of said pair of heater blocks which are relatively moveable towards each other so as to move the first and second elements with pressure towards each other, and that said second ring member is formed of a material having a lower thermal conductivity than a material of said first ring member, and is longer than said first ring member such that said second ring member defines a means for preventing said first and second elements from being moved closer together than a predetermined separation distance so as to predetermine a thickness of the optical element. With this configuration, the mold has the second ring member made of a material with a lower thermal conductivity than that of the first ring member and disposed on the outside of the first ring member. This has the effect that heat is prevented from escaping from the peripheral portion of an optical element and it is difficult for a temperature difference to develop in a glass. Preferably, a more positive effect can be obtained by providing a clearance of 3 mm or less between said first and said second ring members. Further, with said second ring member made longer than said first ring member, the heater blocks come in contact with said second ring member, permitting the thickness of the optical element to be predetermined. At this time, finishing the second ring member with a high precision allows the parallelism of a lens to be predetermined. The properties to be considered in selection of the second ring member material include superior oxidation-resistance, large compressive strength and low cost. In order to satisfy the condition mentioned above, a suitable material may be stainless steel, preferably that of austenitic base or of martensitic base. The first ring member guides said first element and said second element. When a mold having a molding surface is inserted into the first ring member, the first molding surface and the second molding surface become perpendicular to the internal surface of the first ring member thereby allowing the optical axes to be aligned. The present invention further provides a molding method for molding an optical element, the method comprising the steps of: supplying a glass having a viscosity of 10¹¹Pa.s (10¹² poise) or more to a mold according to any one of claims 1 to 7, said mold being at a temperature at or below the glass transition point; heating said glass and a portion of said mold near a molding surface thereof to a temperature corresponding to a glass viscosity of 10⁷ to 10⁹Pa.s (10⁸ to 10¹⁰ poise); pressing said glass for 3 to 90 seconds; cooling said glass at a cooling rate of 1.5 to 2.5°C/sec to a temperature corresponding to a viscosity of 10⁹ to 10¹⁰Pa.s (10¹⁰ to 10¹¹ poise) and thereafter at a cooling rate of 0.2 to 1.5°C/sec while maintaining pressure on the glass; and taking out the molded glass as an optical element at a temperature at or below the glass transition point. The molding method for optical elements according to the present invention is performed within a range 10⁷ to 10 11.75 Pa.s (10⁸ to 10 12.75 poise), so that both the glass and the mold are cooled to prevent deformation in the shape of a molded product and to prevent the development of distortions due to cooling. Also, when under a pressurized condition the cooling rate is controlled such that the temperature of the mold is the same as that of the glass in the cooling process before annealing, no change in surface accuracy is found and there is little residual distortion even though annealing is performed. On the contrary, an optical element having been molded without being processed with such cooling process cannot maintain a shape obtained before the cooling process when annealing is performed. A press molding machine used for the molding method for optical elements is a simplified press molding machine which has no special space for replacing air with inert gas and performs no vacuum exhaust. Figs. 1 (A) and (B) are sectional views of example molds for optical elements according to the present invention. Fig. 2 is a perspective view of a press molding machine used in the embodiments of the present invention, and Fig. 3 is a plan view of the press molding machine used in the embodiments of the present invention. Fig. 4 is a sectional view showing heater blocks and a mold in a first cooling stage used in the embodiments of the present invention. Fig. 5 is a sectional view of cooling blocks and a mold in a rapid cooling stage used in the embodiments of the present invention. Fig. 6 is a glass temperature profile of the molding method for optical elements in Example 1. Referring to drawings, molds for molding optical elements according to the present invention will be explained hereinafter. Figs. 1 (a) and (b)) show sectional views of molds for molding a bi-convex lens. In each drawing, the upper element 1 and the lower element 2 are arranged such that they are guided by a first ring member 6. Further, a second ring member 7 is provided on the outside of the first ring member. The upper and lower heater blocks 16 and 17 are heated by cartridge heaters 16a and 17a as heating sources, respectively. Between the upper element 1 and the lower element 2, a raw glass material 4 for a lens to be molded is held. The raw glass material 4 is heated with the heat produced by the heater blocks, and press molded with the pressure applied from either or both of the heater blocks 16 and 17. In a cooling process after having been heated and press molded, the whole of the lens to be molded requires to be uniformly cooled. Accordingly, the second ring member made of a material with a lower thermal conductivity than that of the first ring member is provided on the outside of the first ring member. The first ring member material is more easily kept at a uniform temperature by using the same material as that of the mold. A material which is suitable for the mold and the first ring member according to the present invention, and which has superior thermal penetrability, is a super hard alloy containing, as a main component, a tungsten carbide (WC) with a thermal conductivity 3.014kW/mK (43.2 kcal/mh°C). Also, a cermet consisting mainly of TiN, TiC or Cr₃C₂, may be used. Further, any other material may be used if its thermal conductivity is 2.093kW/mK (30 kcal/mh°C) or more. At this time, with the clearance between the outer diameter of the first and second elements and the inner diameter of the first ring member made very small to keep the perpendicularity, the optical axes of an optical element can be made aligned with each other. In order to keep a heat insulation effect whilst performing cooling using the second ring member, austenitic base stainless steel with a superior heat resistance, such as SUS316 with a thermal conductivity 0.937kW/mK (13.4 kcal/mh°c) or SUS304 with a thermal conductivity 0.98kW/mK (14.0 kcal/mh°C), and martensitic base stainless steel such as SUS431 with a thermal conductivity 1.224kW/mK (17.5 kcal/mh°C) can be used. Further, with the second ring member made longer than the first ring member and with the end faces of the second ring member made parallel to one another, both lens thickness and the parallelism of the lens faces can be predetermined. Also, when the clearance between the outer diameter of the first ring member and the inner diameter of the second ring member is excessively wide, the heat insulation effect is lost, so that the clearance should be within 5 mm and preferably within 3 mm. The difference between the molds shown in Figs. 1 (a) and (b) is whether the mold has a flange part on this underside or not. Although the mold shown in Fig. 1 (b) is usually preferable for purposes of being transferred, the mold shown in Fig. 1 (a) is preferable to improve cycle time. Referring to the drawings, the method and apparatus for molding optical elements using the mold for optical elements according to the present invention will be explained hereinafter. Fig. 2 is a perspective view of a press molding machine used in the present invention, and Fig. 3 is a plan view of Fig. 2. A mold holding a raw glass material with a viscosity of 10¹¹Pa.s (10¹² poise) or more is fed into a mold feeding port, allowed to slide on a feeding rail 22 by a suitable transfer means (not shown), and fed to a heating stage 25. The raw glass material may preferably be a glass for molding whose shape is configured by a relational expression O.02X < Y < 0.8X where Y is a difference between the diameter of an optical element to be molded and that of the glass for molding, and X is a movement of the mold. The heating stage 25 and a press stage 26 have been heated to a temperature corresponding to a viscosity of the glass of 10⁷ to 10⁹Pa.s (10⁸ to 10¹⁰ poise). After the mold has been sufficiently heated, the mold is allowed to slide on a transfer rail 31 by a transfer member 24, and sent to the press stage 26. Then, with a press cylinder 21 lowered by a pressurizing device 30, the mold is pressed by the upper heater block 16 (Fig. 4). Press pressure is sufficient when it can deform the raw glass material for a short time, preferably 500 kg/cm² or less. After having been pressed for 30 to 90 seconds, with the press cylinder 21 raised, the mold is transferred to a first cooling stage 27 by the transfer member 24. The first cooling stage 27 is set to a temperature lower than that of the press stage. Although a second cooling stage is provided in this embodiment, in some cases only one stage will be necessary. Accordingly, in a pressed condition after having been transferred to these cooling stages, the mold is cooled at a cooling rate 1.5 to 2.5°C/sec for viscosities of 10⁹ to 10¹⁰Pa.s (10¹⁰ to 10¹¹ poise) or less and at a cooling rate 0.2 to 1.5°C/sec for viscosities of 10⁹ to 10¹⁰Pa.s (10¹⁰ to 10¹¹ poise) or more. Finally, the mold whose temperature has become the transition point or less of the glass is transferred to the position between rapid cooling stages 18 and 19 to be cooled (Fig. 5), and is taken out from a taking out port through a taking out rail 23. At this time, a distortion can also be removed by feeding the mold to an annealing furnace (not shown) without transferring to the rapid cooling stages. In a molding chamber 33, nitrogen gas is being blown off through nitrogen gas blow off ports 32, making the inside of the chamber inactive. The transfer member 24 moves such that it moves in the y(-) direction to hold the mold, and moves in the x (-) direction to transfer the mold to the next stage. Thereafter, it moves in the y (+) direction followed by the x (+) direction. With the movement repeated, the whole transferring process can be carried out. Example 1 Press molding was performed using the mold shown in Fig. 1 (a). A glass material to be molded was SF6. First, the raw glass material was fed into a mold at room temperature, and transferred to the heating stage 25. The glass was heated to 520°C (corresponding to a glass viscosity of 10⁷Pa.s (10⁸ poise)) in the heating stage. At this time, although the glass may be heated to a viscosity of 10⁷Pa.s. (10⁸ poise) or more, the heating time becomes long which does not improve lens performance, so that the heating up to viscosity of 10⁷Pa.s (10⁸ poise) is sufficient with respect to cycle time. Then, the glass was transferred to the press stage 25, and, with the press cylinder 21 lowered, pressed for 10 seconds. The press pressure was 10 kg/cm². Then, after the press pressure had been released, the glass was transferred to the first cooling stage 27. After having been transferred to the first cooling stage 27 which was constantly kept at 465°C, the glass was subjected to a press pressure for 30 seconds. At this time, at the cooling rate of 1.5°C/sec, the glass was cooled to 470°C (corresponding to a glass viscosity of 10⁹Pa.s (10¹⁰ poise)). Then, the glass was transferred to the second cooling stage 28 kept at 430°C, and held for 80 seconds in a pressed condition. At this time, the glass became 431°C (corresponding to a viscosity of 10¹²Pa.s (10¹³ poise)), and the cooling rate was 0.5°C/sec. However, at this time, an attempt was made to cool the glass at the cooling rate 1.0°C/sec for viscosities of up to 10⁹Pa.s (10¹⁰ poise) and 0.1°C/sec for viscosities of up to 10¹²Pa.s (10¹³ poise) in order to improve lens performance, with the result that the lens performance was nearly the same and the cooling time for viscosities of up to 10¹²Pa.s (10¹³ poise) increased by 300 seconds, causing cycle time to become long and lens cost to be high. The pressed lens thus cooled was transferred to the rapid cooling stage 29, and cooled rapidly and taken out. The wavefront aberration of the pressed lens thus obtained was highly precise at RMS = 0.025 . Also, in this case, although the mold had no non-contact part not in contact with the heater block, a pressed lens having a very good lens performance was molded by using the second ring element.

Claims: A mold for molding a molding material into an optical element, comprising: first and second elements (1, 2) each of which includes an inner molding surface adapted to contact the molding material and an outer surface opposite said inner molding surface, said inner molding surfaces of said first and second elements defining a cavity therebetween in which the molding material is adapted to be received and molded to form the optical element; a first ring member (6) mounted about said first and second elements and having means, comprising an inner surface thereof, for guiding movement of said first and second elements towards and away from each other; and a second ring member (7) mounted about said first ring member, characterized in that said mold is adapted to be mounted between a pair of heater blocks (16, 17), said outer surface of each of said first and second elements (1, 2) being adapted to contact a respective one of said pair of heater blocks which are relatively moveable towards each other so as to move the first and second elements with pressure towards each other, and that said second ring member (7) is formed of a material having a lower thermal conductivity than a material of said first ring member, and is longer than said first ring member (6) such that said second ring member defines a means for preventing said first and second elements from being moved closer together than a predetermined separation distance so as to predetermine a thickness of the optical element. A mold according to claim 1, wherein said second ring member (7) is adapted to contact the pair of heater blocks (16, 17), respectively, to thereby prevent said first and second elements from being moved closed together than the predetermined separation distance so as to predetermine the thickness of the optical element. A mold according to claim 1, wherein each of said first and second elements (1, 2) includes a flange extending laterally outwardly from said outer surface so as to extend the surface area which is adapted to contact a respective one of the pair of heater blocks, and wherein said second ring member (7) is adapted to contact said flanges of said first and second elements, respectively, to thereby prevent said first and second elements from being moved closer together than the predetermined separation distance so as to predetermine the thickness of the optical element. A mold according to any of claims 1 through 3, wherein said guiding means is operable to predetermine an optical axis of the optical element moulded in said cavity. A mold according to any of claims 1 through 4, wherein each of said first and second elements (1, 2) and said first ring member (6) is made of a super hard alloy containing tungsten carbide (WC) as a main component or a cermet containing one of titanium nitride (TiN), titanium carbide (TiC) or chrome carbide (Cr₃C₂) as a main component. A mold according to any of claims 1 through 5, wherein said second ring member (7) is made of an austenitic-based stainless steel or a martensitic-based stainless steel. A mold according to any of claims 1 through 6, wherein a clearance between an outer diameter of said first ring member (6) and an inner diameter of said second ring member (7) is less than or equal to 3mm. A molding method for molding an optical element, the method comprising the steps of: supplying a glass having a viscosity of 10¹¹Pa.s (10¹² poise) or more to a mold according to any one of claims 1 to 7, said mold being at a temperature at or below the glass transition point; heating said glass and a portion of said mold near a molding surface thereof to a temperature corresponding to a glass viscosity of 10⁷ to 10⁹Pa.s (10⁸ to 10¹⁰ poise); pressing said glass for 3 to 90 seconds; cooling said glass at a cooling rate of 1.5 to 2.5°C/sec to a temperature corresponding to a viscosity of 10⁹ to 10¹⁰Pa.s (10¹⁰ to 10¹¹) poise and thereafter at a cooling rate of 0.2 to 1.5°C/sec while maintaining pressure on the glass; and taking out the molded glass as an optical element at a temperature at or below the glass transition point.

Inventors: INOUE TAKASHI, KIMOTO TAKAYUKI, NAKAMURA SHOJI, SUNOHARA MASAAKI, YONETANI DAIJIROU, INOUE, TAKASHI, KIMOTO, TAKAYUKI, NAKAMURA, SHOJI, SUNOHARA, MASAAKI, YONETANI, DAIJIROU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: C03B11
IPC codes: B29C43, C03B11