Machine for molding optical element and method of producing the optical element by using the machine

Abstract: A machine for molding an optical element by using a molding block having upper and lower molding dies, a sleeve die and a glass blank combined integrally with each other, comprising: a chamber which accommodates a heating zone of a plurality of sections for heating the molding block from above and below, a deformation zone for pressing the molding dies and a cooling zone of a plurality of sections for cooling the molding block and is formed with an inlet and an outlet for the molding block; the sections of the heating zone, the deformation zone and the sections of the cooling zone being provided independently of one another; a transport member for sequentially transporting the molding block through the heating zone, the deformation zone and the cooling zone; first and second shutters for opening and closing the inlet and the outlet, respectively; and a gas supply member for introducing nonoxidizing gas into the chamber, and a method of producing the optical element by using the machine.

Description: The present invention relates to a molding machine for mainly press molding a high precision optical element and a method of producing the optical element by using the molding machine. Recently, in production of an optical element represented by an optical lens, a number of attempts have been made in which the optical element is obtained by press molding through cancellation of a polishing process. Thus, at present, press molding of the optical element is set in a stage of mass production in several optical lens makers. In the most efficient one in known press molding methods of the optical element, molten glass material is poured into a die and then, is press molded. However, in this known method, it is difficult to accurately control contraction of glass during cooling of glass and thus, a high precision optical element cannot be produced efficiently. This is because temperature difference in glass between a melting point of about 1,300 °C at which glass is molten and a transition point at which glass is set is too large. Therefore, an extremely large period is required for cooling glass. Thus, even if a high precision optical element is obtained, this known method is problematical in view of its efficiency. Accordingly, in a general known molding method, a glass blank is preliminarily worked into a certain shape and then, is supplied between molding dies so as to be press molded through heating as described in, for example, Japanese Patent Laid-Open Publication Nos. 58-84134 (1983) and 60-200833 (1985). Japanese Patent Laid-Open No. 61-26528 (1986) discloses a machine for pressing a lens, which is constituted by an inlet chamber for a glass preform, a heating chamber, a pressing chamber, a gradual cooling chamber and an outlet chamber for the lens. In this known machine, the machine housing as a whole should be adapted to be evacuated to vacuum and a number of molding dies are employed so as to raise production rate. However, this known machine is quite expensive because the machine housing as a whole should be evacuated to vacuum. Furthermore, since a number of the molding dies machined with high precision are employed, production cost of the lens rises considerably. Meanwhile, Japanese Patent Laid-Open Publication No. 62-292629 (1987) discloses a machine in which processes of heating, pressing and cooling molding dies can be performed in an identical chamber and the molding dies are heated efficiently from above and below. However, productivity of this prior art machine is low due to its intermittent transport of the molding dies. Furthermore, this prior art machine has a drawback that a large uniform heating portion of a heat source cannot be obtained. Japanese Patent Laid-Open Publication Nos. 62 -292636 (1987) and 2-34526 (1990) discloses a machine in which molding dies are heated and cooled from one side. Thus, the conventional machine has such problems that the molding dies cannot be heated and cooled efficiently and that since temperature difference occurs in a vertical direction of the molding dies, lenses having stable performance cannot be obtained. Furthermore, U.S. Patent No. 4,913,718 discloses a machine in which a glass blank and molding dies are separately heated on an identical pallet and the glass blank in a high-temperature state is supplied into the molding dies so as to be press molded. However, this known machine becomes structurally complicated and expensive due to handling of the glass blank at high temperature, etc. Furthermore, U.S. Patent No. 4,836,838 and U.S. Patent No. 4,915,720 disclose an apparatus for molding glass articles, a plurality of processing chambers including a heating chamber, a pressing chamber and a cooling chamber arranged in order along a predetermined cycler transport pass and a method for molding glass articles. However, this known apparatus and method becomes structurally complicated. As is seen from the foregoing, the prior art documents do not give detailed description of temperature distribution and uniform heating of the glass blank, the molding dies, the heat source, etc., on which performance of the molded item directly relies on. SUMMARY OF THE INVENTION Accordingly, an essential object of the present invention is to provide not only an inexpensive and highly efficient molding machine for molding an optical element, which is simple in structure and is capable of performing remarkably uniform heating but also a highly efficient method of producing the optical element. In order to accomplish this object of the present invention, a machine for molding an optical element by using a molding block in which upper and lower molding dies, a sleeve die for supporting the molding dies and a glass blank are integrally combined with each other is provided, which comprises: a chamber which accommodates a heating zone for heating the molding block from above and below, a deformation zone for pressing the molding dies of the molding block and a cooling zone for cooling the molding block and is formed with an inlet for loading the molding block into the chamber and an outlet for discharging the molding block from the chamber; the heating zone including a plurality of sections, while the cooling zone includes a plurality of sections such that the sections of the heating zone, the deformation zone and the sections of the cooling zone are controllable independently of one another; a transport means for sequentially transporting the molding block through the heating zone, the deformation zone and the cooling zone; first and second shutters for opening and closing the inlet and the outlet, respectively; and a gas supply means for introducing nonoxidizing gas into the chamber. Furthermore, the invention provides a method of producing an optical element, by the above machine, comprising: the first step of integrally combining upper and lower molding dies, a sleeve die for supporting the molding dies and a glass blank with each other so as to obtain a molding block; the second step of heating the molding block from above and below in one or more steps; the third step of pressing the glass blank through the molding dies in one or more steps so as to deform the glass blank; and the fourth step of cooling the molding block from above and below in one or more steps; the second to fourth steps being continuously performed in nonoxidizing atmosphere in an identical chamber. In the present invention, since a plurality of independent temperature control means and pressure control means are provided in the chamber, the molding block having a predetermined heat capacity can be accurately heated, pressed for deformation and cooled uniformly at extremely high thermal efficiency in a predetermined period. In accordance with the machine of the present invention, since the glass blank can be heated, pressed for deformation and cooled at high thermal efficiency, the highly accurate optical element can be press molded in a short period. Meanwhile, the machine is made simple in structure and compact in size. Furthermore, since hermetic sealing is not required in the machine, the machine is suitable for mass production at low cost. Meanwhile, in accordance with the method of the present invention, the optical element having highly accurate shape can be produced at low cost. As a result, an optical apparatus incorporating the optical element can be operated stably and produced at low cost. BRIEF DESCRIPTION OF THE DRAWINGS This object and features of the present invention will become apparent from the following description taken in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, in which: Fig. 1 is a sectional front elevational view of a machine for molding an optical element, according to one embodiment of the present invention; Fig. 2 is a sectional top plan view of the machine of Fig. 1; Fig. 3 is a sectional side elevational view of the machine of Fig. 1; Fig. 4 is a perspective view of a lower heating plate employed in the machine of Fig. 1; Fig. 5a is a fragmentary sectional view of heat insulating plates employed in the machine of Fig. 1; Fig. 5b is a graph indicative of effects of the heat insulating plates of Fig. 5a; Fig. 6 is a perspective view showing each heating plate and heaters employed in the machine of Fig. 1; Fig. 7 is a perspective view of each of the heat insulating plates of Fig. 5a; Fig. 8 is a fragmentary sectional view explanatory of an upper heating plate employed in the machine of Fig. 1; Figs. 9a and 9b are views showing a molding block used in a method of producing an optical element, according to the present invention; Fig. 10 is a graph showing temperature curve of the molding block of Fig. 9a; and Figs. 11a, 11b and 11c are sectional views showing glass blanks used in the method of the present invention, respectively. Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout several views of the accompanying drawings. DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings, there is shown in Fig. 1, a machine K for molding an optical element, according to one embodiment of the present invention. The machine K includes a chamber 1 in which a base 2 is accommodated. Lower heating plates 3-9 are securely mounted on the base 2 so as to be arranged in a line extending in the X-axis direction in Fig. 1. Each of the lower heating plates 3-9 is provided with a pair of heaters 22 acting as a temperature control means. Meanwhile, upper heating plates 10-16 are provided above the lower heating plates 3-9 so as to confront the lower heating plates 3-9, respectively. Each of the upper heating plates 10-16 is also provided with a pair of the heaters 22 acting as a temperature control means and a press rod 21 of a cylinder, which acts as a pressure control means. The upper heating plates 10-16 and the lower heating plates 3-9 are spaced a distance which allows a molding block to pass therethrough. The molding block is constituted by an upper die 17, a lower die 18, a sleeve die 19 for supporting the upper and lower dies 17 and 18 and a glass blank 20 gripped between the upper and lower dies 17 and 18. A heating zone for heating the pressing block is formed by the lower heating plates 3-5 and the upper heating plates 10-12. A pressing deformation zone for deforming the glass blank 20 is formed by the lower heating plate 6 and the upper heating plate 13. Meanwhile, a cooling zone for cooling the molding block is formed by the lower heating plates 7-9 and the upper heating plates 14-16. Each of the upper heating plates 10-16 is arranged to travel through a predetermined stroke vertically, i.e. in the Z-axis direction in Fig. 1. A pair of the heaters 22 is embedded in each of the heating plates 3-16 so as to enable desired temperature rise of the heating plates 3-16. A cooling circuit is incorporated in the base 2 and has a cooling water inlet 23 and a cooling water outlet 24 which are connected with an external temperature controller (not shown). Although not specifically shown, a cooling water is incorporated in an outer wall of the chamber 1 so as to prevent temperature rise of the outer wall of the chamber 1 such that safety of molding operation is ensured. In this embodiment, the number of the lower heating plates 3-9, i.e. the number of the upper heating plates 10-16 is set to 7. The reason why the 7 lower heating plates (upper heating plates 10-16) are provided will be described later. Furthermore, a thermocouple 44 (Fig. 3) is embedded in the heating plates 3-16 so as to detect and control temperature of the heating plates 3-16. In order to control atmosphere in the chamber 1, two gas inlets 25 for introducing nonoxidizing gas into the chamber 1 are formed at opposite ends of an upper portion of the chamber 1, respectively. In order to enable transport of the molding block on the lower heating plates 3-9, upper faces of the lower heating plates 3-9 are made flush with each other. An inlet 26 and an outlet 29 which enable pass of the molding block therethrough are, respectively, provided at opposite ends of the chamber 1. The lower heating plate 3 is connected, through the inlet 26, with exterior of the chamber 1 by a loading platform 27. A shutter 28 is vertically movably provided at the inlet 26 so as to open and close the inlet 26. On the other hand, the lower heating plate 9 is connected, through the outlet 29, with exterior of the chamber 1 by a discharge platform 31. A shutter 30 is vertically movably provided at the outlet 29 so as to open and close the outlet 29. When the molding block is pressed through a predetermined distance in the X-axis (-) direction in Fig. 1 by a push rod 33 of a cylinder 32, the molding block is transported onto the lower heating plate 3 in the chamber 1. Fig. 2 shows a transport device 34 for transporting the molding block among the lower heating plates 3-9. The transport device 34 includes drive cylinders 35 and 36 provided at a rear portion of the chamber 1 and a pusher 37 having a comb-like shape. The molding block is transported through a distance identical with a pitch P of the heating plates 3-16 in the X-axis direction and a predetermined distance in the Y-axis direction by the pusher 37 upon actuation of the drive cylinders 35 and 36. The drive cylinder 35 is attached to a cylinder housing H and the drive cylinder 36 is fixed to a rear face of the chamber 1 so as to displace the cylinder housing H in the X-axis direction. The cylinder housing H is slidably mounted on the rear face of the chamber 1 so as to be slightly sealed relative to the rear face of the chamber 1. Figs. 3 and 4 show detailed arrangements of the heating plates 3-16, which are the most essential constituent elements of the present invention. The heating plates 3-16 have an identical rectangular shape. Thus, by way of example, the lower heating plate 3 and the upper heating plate 10 are shown in Figs. 3 and 4. As shown in Figs. 3 and 4, a heat insulating plate 41a and the lower heating plate 3 are securely piled on the base 2 in this order. Furthermore, a pair of guides 39 for positioning the molding block in the Y-axis direction in Fig. 3 are secured to the heat insulating plate 41a. By using the transport device 34 referred to above, the molding block is slidably moved between the guides 39 and on one lower heating plate to the next lower heating plate. As shown in Fig. 4, four side faces and a bottom face of the lower heating plate 3 are surrounded by heat insulating plates 41b-41e and the heat insulating plate 41a, respectively. The heat insulating plates 41a-41e are provided for such purposes that (1) effect on the lower heating plate by even minute temperature change of the cooling circuit incorporated in the base 2 is eliminated, (2) accurate temperature control of each lower heating plate can be performed independently of the two neighboring lower heating plates and (3) heat dissipation from the lower heating plate 3 through convection of nonoxidizing gas in the chamber 1 is prevented so as to uniformly heat the molding block. One vital point in molding of an optical element is how uniform heating is obtained during heating and cooling of the molding block including the glass blank 20. To this end, temperature difference between surfaces of the upper and lower heating plates should be reduced so as to increase a uniform heating area. Figs. 5a and 5b show uniform heating property achieved by the heat insulating plates 41a-41e. Namely, Fig. 5b shows temperature of surface of the lower heating plate 3 when the lower heating plate 3 is set to 580 °C by removing the heat insulting plate 41b of Fig. 4 from the lower heating plate 3 as shown in Fig. 5a. Temperature of surface of the lower heating plate 3 is measured by an infrared radiation thermometer as shown by the solid line in Fig. 5b. At the left portion in Fig. 5b, temperature of surface of the lower heating plate 3 is about 20 °C lower than the set temperature of 580 °C, thus resulting in improper uniform heating property. This improper uniform heating property is mainly caused by heat dissipation from surface of the lower heating plate 3. Meanwhile, the broken line in Fig. 5b shows uniform heating property obtained by mounting the heat insulating plate 41b on the lower heating plate 3 and is about 4 °C lower than the set temperature of 580 °C, thereby resulting in substantially uniform heating property. Such uniform heating property should be secured in a large area of the lower heating plate, with which the lower die 18 comes into contact. Examination of uniform heating property in the above mentioned area of the lower heating plate 3 based on a similar measuring method has revealed that uniform heating property similar to that of the broken line of Fig. 5b is obtained. On the other hand, the upper heating plates 10-16 are arranged in manner basically similar to that of the lower heating plates 3-9. In Fig. 3, a cooling plate 46 is secured to a lower face of a flange 45 of the press rod 21 with screws. A cooling circuit (not shown) is provided in the cooling plate 46 and includes metal pipes 47 and 48 which are fixed to the cooling plate 46 with screws. The pipe 48 is connected with an external temperature controller (not shown). Temperature adjusting water is poured into the pipe 47 so as to be discharged from the pipe 48. The pipes 47 and 48 are vertically slidably provided so as to be vertically displaced upon vertical travel of the press rod 21. A mounting plate 49 is mounted on a lower face of the cooling plate 46 with screws. Furthermore, in the same manner as in the lower heating plate 3, the upper heating plate 10 is fixed, through a heat insulating plate 50a, to the mounting plate 49 with screws and four side faces of the upper heating plate 10 are surrounded by heat insulating plates 50b-50e, respectively such that effects identical with those of the lower heating plate 3 can be achieved by the upper heating plate 10. Since the heating plates 3-16 are provided independently of each other as described above, replacement of the heating plates 3-16 during maintenance due to lapse of service life of the heaters 22, etc. is facilitated. In Fig. 3, the mounting plate 49 is used not only for securing the upper heating plate 10 and the heat insulating plate 50a but for relaying lead wires 43 of the heaters 22 and the thermocouple 44. The lead wires 43 and the thermocouple 44 drawn from an end face of the upper heating plate 10 are passed through a sleeve 50 which is secured to the mounting plate 49 with a screw 52. At an upper end of the sleeve 50, the lead wires 43, the thermocouple 44 and the sleeve 50 are fixed to one another by a fastener 51. In the same manner as in the pipes 47 and 48, the sleeve 50 is vertically slidably provided so as to be vertically displaced upon vertical travel of the press rod 21. As described above, the pipes 47 and 48 and the sleeve 50 having the lead wires 43 and the thermocouple 44 extending therethrough are slidably provided for the following reason. Namely, since the lead wires 43 are exposed to rather high temperature in the vicinity of the upper heating plate 10 having the heaters 22 embedded therein and bending stress due to vertical travel of the press rod 21 is applied to the lead wires 43, breaking of the lead wires 43 is likely to take place unless the pipes 47 and 48 and the sleeve 50 are slidably provided. The wires 43 are held in the movable sleeve 50 in the chamber 1 and bending of the wires 50 due to vertical movement of the sleeve 50 can be absorbed outside the chamber 1 at a large radius of curvature at room temperature. Therefore, breaking of the wires 43 is least likely to occur, thereby resulting in remarkable improvement of reliability of the machine K. Fig. 6 shows relation between each of the heating plates 3-16 and the heaters 22. This arrangement also aims at improving uniform heating property of the heating plates 3-16. Since the heating plates 3-16 have the identical arrangement, only the lower heating plate 3 is described, hereinbelow. The heating plate 3 is made of metal having excellent heat resistance and oxidation resistance, for example, stainless steel SUS316 of JIS (Japanese Industrial Standards) corresponding to ASTM A270-73. Two through-holes 53 are formed in the heating plate 3 and the cylindrical cartridge type heater 22 is inserted into each of the through-holes 53 via abrasive grains 54. In order to obtain the abrasive grains 54 in this embodiment, powder in which zirconium oxide is mixed with aluminum oxide, silicon carbide, etc. having uniform grain size suitable for lapping is dissolved in water and then, filled in a gap between the through-hole 53 and the heater 22 so as to be dried. The abrasive grains 54 are filled in the gap between the through-hole 53 and the heater 22 so as to achieve the following effects (1) to (3). (1) The gap between an outer peripheral surface of the heater 22 and a peripheral surface of the through-hole 53 is made uniform such that heat conduction from the heater 22 is uniformed. (2) Seizure between the heater 22 and the heating plate 3 is prevented and replacement of the heater 22 at the time of breaking of the lead wire 43 is facilitated. (3) Error in outside diameter of the heater 22 and machining error of the through-hole 53 are absorbed. By the effects (1) to (3), uniform heating property and easy maintenance of the heating plates 3-16 can be secured. Uniform heating property of the heating plates 3-16 is degraded by deviation of the heater 22 from a central position in the through-hole 53 to a degree substantially identical with that of a case of absence of the heat insulating plates. Meanwhile, in order to absorb machining error of the through-hole 53, grain size of the abrasive grains 54 may be selected in accordance with the gap between the heater 22 and the through-hole 53. In a conventional arrangement in which ceramic adhesive is filled in the gap between the heater and the heating plate, the above effects (1) and (3) can be achieved but the effect (2) cannot be obtained at all. Thus, the above arrangement of the present invention secures not only uniform heating property but easy maintenance. Accordingly, when the arrangements of Figs. 5 and 6 are combined with each other, the upper and lower heating plates 3-16 exhibit most excellent uniform heating property. Fig. 7 shows the heat insulting plate 41a provided under the lower heating plate 3. The heat insulating plate 41a is formed with two screw holes 55 for mounting the heat insulating plate 41a on the base 2 and three mounting holes 56 for mounting the heat insulating plate 41a on the heating plate 3. Characteristics required of the heat insulating plate 41a are long service life including heat insulation, heat resistance, thermal shock resistance and pressure resistance and excellent machining property. In this embodiment, the heat insulating plate 41a is formed by a vitrified wheel mainly consisting of aluminum oxide and silicon oxide for the following grounds. Namely, commercially available ceramic heat insulating material satisfies the above mentioned characteristics at an initial stage. However, in the machine K, the heat insulating plate 41a is subjected to frequent rise and drop of temperature and is pressed repeatedly. Furthermore, thermal shock is applied to the heat insulating plate 41a in the direction of thickness of the heat insulating plate 41a due to temperature difference of about 600 °C. Thus, if the heat insulating plate 41a is made of ceramic insulating material, service life of the heat insulating material 41a becomes extremely short. In this embodiment, the heat insulating plate 41a is formed by a wheel which is made of 78 % of aluminum oxide, 20 % of silicon carbide, 1.5 % of potassium oxide, 1 % of sodium oxide and 0-0.5 % of calcium oxide and has a porosity of 39.5 % and a heat resistance temperature of 1,300 °C. For comparison, the heat insulating plate 41a formed by the above wheel and a conventional heat insulating plate made of commercially available ceramic insulating material are mounted on the machine K. Evaluation of their service life has shown that service life of the heat insulating plate 41a is quite different from that of the conventional heat insulating plate after operation of the machine K for one month. Minute cracks are produced in the conventional heat insulating plate so as to prevent the conventional heat insulating plate from maintaining its original form. On the other hand, substantially no crack is found in the heat insulating plate 41a even after operation of the machine K for one year. Generally, most of heat insulating materials contain gas (air in many cases) in solid and are based on a principle that coefficient of thermal conductivity of gas is lower than that of solid. Since the wheel having the components suitable for heat insulation and the large porosity is excellent heat insulating material and is thermally stable, the heat insulating plate 41a is excellent in view of its service life. Meanwhile, in this embodiment, the heat insulating plate 41a is mainly made of aluminum oxide. However, even if the heat insulating plate 41a is mainly made of silicon carbide, the heat insulating plate 41 exhibits the same effects as those of the heat insulating plate 41a made mainly of aluminum oxide. Furthermore, the wheel which has been conventionally used as a grinding tool is used as high-temperature heat insulating material, application field of the wheel is expanded. Fig. 8 shows each of the upper heating plates 10-16, for example, the upper heating plate 10. A release pin 57 for releasing the heating plate 10 from the upper die 17 is embedded at a central portion of the heating plate 10. Since a lower face of the heating plate 10 and an upper face of the upper die 17 are precisely machined so as to be brought into contact with each other, the upper die 17 is lifted upwardly by suction or close contact with the heating plate 10, especially at the time of completion of deformation of the glass blank 20, molding of the glass blank 20 cannot be performed properly unless the release pin 57 is provided. The release pin 57 is urged by a compression spring 58 so as to project out of the lower face of the heating plate 10 through about 1 mm. Suction and close contact of the upper die 17 with the heating plate 10 is forcibly eliminated by the compression spring 58. An urging force of the compression spring 58 is set at such a value as not to degrade performance of the molded lens, for example, 200 g-f approximately. Meanwhile, in this embodiment, the urging force of the compression spring 58 is employed for releasing the heating plate 10 from the upper die 17. However, this arrangement can be replaced by a method in which grooves are formed on the lower face of the heating plate 10 so as to reduce a contact area between the lower face of the heating plate 10 and the upper face of the upper die 17 or a method in which gas pressure is employed for releasing the heating plate 10 from the upper die 17. Hereinbelow, a method of producing, by using the machine K, an optical element represented by a lens, in another aspect of the present invention is described with reference to Figs. 1-2 and 9-11 and Table 1. The molding block is obtained by inserting the glass blank 20 in the upper die 17, the lower die 18 and the sleeve die 19 and is placed on the loading platform 27. In Fig. 2, the molding block disposed at the position A on the discharge platform 31 is displaced to the position B in the direction of the arrow by a piston 38 of a cylinder and then, the piston 38 is returned to its original position. Subsequently, by driving the drive cylinder 35 through a predetermined distance in the Y-axis (-) direction simultaneously with opening of the shutter 30, the pusher 37 is pushed in the Y-axis (-) direction such that each of teeth of the pusher 37 is inserted in between a pair of the neighboring molding blocks placed on the heating plates 3-9. Thereafter, the drive cylinder 36 is driven in the X-axis (-) direction such that each of the molding blocks on the heating plates 3-9 is transported to the next one of the heating plates 3-9. For example, the molding block on the heating plate 9 for cooling is passed through the outlet 29 so as to be transported onto the discharge platform 31. Then, the pusher 37 is returned to its original position in the order of the Y-axis (+) direction and X-axis (+) direction and, at the same time, the shutter 30 is closed. Subsequently, the shutter 28 of the inlet 26 is opened and the molding block on the loading platform 27 is displaced in the X-axis (-) direction by the push rod 33 of the cylinder 32 so as to be transported onto the heating plate 3 in the chamber 1. Simultaneously with return of the push rod 33 to its original position, the shutter 28 is closed. These actions are sequentially repeated at a predetermined interval such that molding of the lens is performed continuously. During opening of the shutters 28 and 30, nonoxidizing gas (e.g., N 2 gas) having a predetermined pressure is supplied into the chamber 1 from the gas inlets 25 so as to raise internal pressure of the chamber 1. During opening of the inlet 26 and the outlet 29 or through gaps of the cylinders, the gas flows out of the chamber 1 but does not adversely affect molding of the lens. This is because at the time of loading and discharge of the molding block, either one of the shutters 28 and 30 provided at the opposite ends of the chamber 1 is closed for the following reason. Namely, if both of the shutters 28 and 30 are opened simultaneously, balance of internal pressure of the chamber 1 is destroyed, so that external air flows into the chamber 1 and thus, it is impossible to perform molding of the lens in stable atmosphere. In case both of the shutters 28 and 30 are opened simultaneously, concentration of oxygen assumes a maximum of 0.1 % or more and varies greatly synchronously with opening and closing of the shutters 28 and 30. On the other hand, if either one of the shutters 28 and 30 is closed, concentration of oxygen usually assumes 0.04 % or less stably. At the same time when the molding block transported into the chamber 1 is placed on the heating plate 3, the heating plate 10 is lowered so as to come into contact with the upper die 17 of the molding block such that the molding block is heated from above and below efficiently. In the heating zone, a plurality of the lower heating plates 3-5, i.e, a plurality of the upper heating plates 10-12 are provided for the following purposes (1) to (3). (1) Before glass of the glass blank 20 reaches a deformation temperature, the upper and lower dies 17 and 18, the sleeve die 19 and the glass blank 20 having different heat conductivities are uniformly heated from above and below without vertical temperature difference. (2) Temperature of the upper and lower heating plates, which has been lowered by the molding block, is returned to the heating temperature in a predetermined period. (3) External appearance of the molded lens is determined by substances scattered from glass of the glass blank 20. In order to prevent scatter of the substances from the glass blank 20, conditions for a minimum deformation temperature of glass are obtained. To this end, heat capacity and electric power of the upper and lower heating plates as well as coefficient of thermal conductivity, heat capacity, etc. of the molding block are, needless to say, calculated. In Fig. 2, when the pusher 37 is displaced in the Y-axis (-) direction and then, in the X-axis (-) direction through a predetermined pitch, the molding block placed on one lower heating plate is transported along the guides 39 through the predetermined pitch in the X-axis (-) direction so as to be placed on the next lower heating plate. Subsequently, the pusher 37 is retracted in the Y-axis (+) direction and then, is returned in the X-axis (+) direction through the predetermined pitch, whereby transport of the molding block is completed. In the heating zone, the molding block is set in waiting state on the lower heating plates 3-5 for a predetermined period so as to be heated from above and below by the upper and lower heating plates 10-12 and 3-5 during the predetermined waiting period. In the heating zone, the molding block is sequentially transported at a predetermined interval so as to be heated. Thus, when the molding block has reached such a temperature as to enable its deformation, heating of the molding block is completed. Subsequently, the molding block is transported onto the heating plate 6 of the pressing deformation zone by the transport means 34 referred to earlier. In the pressing deformation zone, the lower heating zone 6 and the upper heating zone 13 are also set to such a temperature as to enable its deformation. Upon transport of the molding block onto the lower heating plate 6, the upper heating plate 13 is pushed in the Z-axis (-) direction by the push rod 21 so as to come into contact with the sleeve die 19 such that the glass blank 20 is deformed. Upon lapse of a predetermined period after completion of deformation of the glass blank 20, the press rod 21 is lifted in the Z-axis (+) direction. Thereafter, the molding block is transported onto the lower heating zone 7 of the cooling zone. Then, the upper heating zone 19 is brought into contact with the heating zone such that the molding block is uniformly cooled from above and below. Subsequently, the molding block is transported onto the lower heating plate 8 so as to be further cooled. Geometrical accuracy of the molded lens is determined by especially how satisfactorily the glass blank 20 is cooled in the vicinity of a transition point of glass at the above mentioned two cooling steps on the lower heating plates 7 and 8. In this embodiment, the second cooling step, namely, the heating plates 8 and 15 are set at a temperature lower than the transition point of glass. Cooling speed of the molding block is high immediately after transport of the molding block onto the lower heating plate 8 but gradually drops with lapse of time. Thus, temperature of the molding block passes through the transition point of glass in a predetermined period and the molding block is further cooled to reach a temperature identical with that of the upper and lower heating plates 15 and 8. Moreover, the molding block is rapidly cooled by the heating plates 9 and 16 to such a temperature as to enable its discharge. After the three cooling steps on the lower heating plates 7-9, the molding block is carried onto the discharge platform 31. As described above, the molding block is sequentially fed through a total of seven steps of heating, pressing deformation and cooling at a predetermined interval, whereby molding of the lens is completed. In this embodiment, a pair of the upper and lower heating plates are set at a predetermined temperature and production rate is set such that temperature difference between the molding block and the upper and lower heating plates becomes zero in a predetermined period. Setting of production rate and a series of operations such as press timing, shutter timing, transport speed, etc. can be performed by a conventional sequence program. As a total number of the cooling, pressing deformation and cooling steps is increased, production rate can be raised in the molding machine and the production method of the present invention. However, in this case, such a problem arises that the number of the heating plates increases. Therefore, in the molding machine and production method of the present invention, an optimum number of the cooling, pressing deformation and cooling steps is determined in consideration of a desired size of the molded lens, accuracy required of the lens, the number of the lenses to be produced, thermal characteristics of material of glass of the lens, etc. Figs. 9a and 9b show the molding block subjected to molding of the present invention. The molding block is constituted by the cylindrical upper and lower dies 17 and 18, the sleeve die 19 receiving the upper and lower dies 17 and 18 and a molded lens 59. Each of the upper and lower dies 17 and 18 has an optically functional face D formed on its one face confronting the glass blank 20. The sleeve die 19 has an outside diameter of 28 mm, while the upper and lower dies 17 and 18 which are fitted into the sleeve die 19 have a diameter of 20 mm. The molding block has a height of 20 mm. The sleeve die 19 serves not only to bring an optical axis of the optically functional face D of the upper die 17 into alignment with that of the lower die 18 but to determine central thickness of the molded lens 59. In order to stabilize geometrical accuracy of the molded lens 59, it is desirable that the glass blank 20 is molded within the optically functional face D of the upper and lower dies 17 and 18 on the ground that a heat insulating layer 60 is formed on an outer periphery of the molded lens 59 during cooling. Therefore, regardless of whether the molded lens 59 is a spherical lens or an aspherical lens, the outer periphery of the molded lens 59 is peeled so as to obtain a core portion of the molded lens 59 such that the the molded lens 59 has a predetermined outside diameter. As shown in Fig. 9b, if the sleeve die 19 is enclosed by a second sleeve die 61 made of material having a small coefficient of thermal conductivity, geometrical accuracy of the molded lens 59 is further stabilized. The second sleeve die 61 is provided for preventing the sleeve die 19 from being cooled faster than an outer peripheral surface of the molding block and may be made of material having an excellent heat accumulating property. Fig. 10 shows temperature curve of the molding block used in this embodiment. In the heating zone, the lower heating plates 3-5, i.e., the upper heating zones 10-12 are set at heating temperatures of 450 °C, 550 °C and 570 °C, respectively. In the pressing deformation zone, each of the upper and lower heating plates 13 and 6 is set at a deformation temperature of 580 °C. In the cooling zone, the lower heating plates 7-9, i.e., the upper heating plates 14-16 are set at cooling temperatures of 540 °C, 480 °C and 60 °C, respectively. In Fig. 10, the molding block is set at each pair of the upper and heating plates for 53 sec. The molding block is sequentially heated at the steps 1-3 and the glass blank 20 is subjected to pressing deformation at the step 4. As is seen from the portion C in Fig. 10, deformation lasts for about 10-20 sec. in 53 sec. and the rest of 53 sec. is used for performing uniform heating of the molding block as a whole. Then, the molding block is cooled at the steps 5-7. At each of the steps 5-7, cooling speed is low at an initial stage but thereafter, assumes a functional cooling curve. Temperature of the deformed glass blank 20 passes through the transition point of glass at the step 6 and then, the glass blank 20 is cured such that the molded lens 59 is obtained. At the step 7, the molding block is rapidly cooled and the molding block is disassembled. Thus, a number of the molded lenses 59 having desired performances are obtained. The obtained molded lens 59 is a double convex lens having an outside diameter of 18 mm and a central thickness of 2.9 mm and opposite convex faces of the molded lens 59 are aspherical. The molded lens 59 has a remarkably high geometrical accuracy of + 1.0 µm or less with respect to a desired design value. Meanwhile, desired temperature curve of the molding block can be obtained by arbitrarily combining production rate, temperature of each of the heating, pressing deformation and cooling zones and the number of steps in each of the heating, pressing deformation and cooling zones in accordance with heat capacity of the upper and lower dies 17 and 18 and the sleeve die 19 and thermal characteristics of the glass blank 20. For example, in the case where a molding block having a heat capacity smaller than that of the above mentioned molding block is molded, the molding block may be heated at the steps 1 and 2, deformed at the step 3 and cooled at the steps 4-7. Furthermore, in this embodiment, each pair of the upper and lower heating plates are set at an identical temperature on the following ground. Namely, in case the molding block is cooled from above and below at an identical cooling speed in the cooling zone, molding free from warpage can be usually performed when opposite faces of the lens have an identical shape. However, when opposite faces of the lens have quite different shapes, for example, in the case of a meniscus lens, a flat-convex lens, etc., warpage of the lens is conspicuous even if the lens is cooled from above and below at an identical cooling speed. If the lens is subjected to warpage in one direction in such a case, temperature conditions are set such that one face of the lens opposite to the direction of warpage should be cooled slowly. Namely, by providing time difference in cooling between upper and lower portions of the lens at the time of curing of the lens, the lens can be molded through control of warpage. Time difference in cooling between upper and lower portions of the lens can be provided in methods (1) in which each pair of the upper and lower heating plates set at different temperatures, respectively are brought into contact with the molding block so as to cool the molding block through heat conduction and (2) in which a gap is formed between the molding block and the upper heating plate such that one face of the molding block confronting the gap is radially cooled. In the method (1), heat conduction between the molding block and each heating plate is inclined to become nonuniform slightly due to stain, an oxidized film on contact faces of the molding block and each heating plate. Thus, in this embodiment, at the step 6 in which temperature of the molding block passes through the transition point of glass, most vital for cooling, a gap of 1 mm is formed between the molding block and the upper heating plate 15 and molding is performed by employing the molding block of Fig. 9 and temperature curve of Fig. 10. As a result, an upward warpage of 3 µm is produced in the molded lens 59. By minimizing warpage of the molded lens 59, it becomes possible to obtain the highly accurate molded lens 59 having a geometrical error of 1 µm or less. Figs. 11a to 11c show the glass blank 20 employed in this embodiment. The glass blank 20 is made of barium borosilicate. The glass blank 20 of Fig. 11a is a spherical lens obtained by ordinary polishing. The glass blank 20 of Fig. 11b is obtained by directly receiving molten glass on a die and cooling the molten glass for its curing. The above mentioned aspherical lens is obtained by washing and molding the glass blank 20 of Fig. 11a or 11b. However, in this case, ordinary molding offers the following problems (1) and (2). (1) A substance having a low vapor pressure, (borate in this embodiment) is scattered from the glass blank 20 so as to adhere to the die and the lens. (2) Due to difference of coefficient of thermal conductivity between the die made of metal for general use and the glass blank 20 made of inorganic material, surface of the glass blank 20 is rapidly heated and thus, minute foaming occurs. Owing to the above described two problems, transmittance is lowered and quality of the molded lens 59 is degraded, so that the molded lens 59 is observed as being tarnished. The glass blank 20 of Fig. 11c is constituted by an excessive layer 62 (hereinbelow, referred to as a "rich layer") formed on its surface and a bulk layer 63. The rich layer 62 has compositions of a high melting point, while the bulk layer 63 has essential compositions of glass. The glass blank 20 of Fig. 11c is obtained by chemically processing the spherical lens of Fig. 11a. This chemical processing is performed by washing the spherical lens of Fig. 11a in ultrasonic wave by using commercially available detergent mainly containing ammonium citrate and having a pH of 7. Table 1 below shows chemical processing period, thickness of the rich layer 62, external appearance of the molded lens and molding property in the glass blank 20 of Fig. 11c. Thickness of the rich layer 62 is obtained by qualitatively examining constituents of glass in the direction of depth by using an Auger electron spectral analyzer. The rich layer 62 has a high melting point and is made of silicon and oxygen free from not only boric acid but barium. In Example a of Table 1, only ordinary precision washing is performed and the rich layer 62 has a thickness of 100 Å or less. In the molded lens of Example a, slight foaming and deposit are observed. In the molded lens of Example e, sufficient processing property cannot be obtained. Example Chemical processing period (min.) Thickness of rich layer (Å) External appearance Molding property foaming deposit a 0 ≦ 100 II or III II I b 1 ≦ 200 I I I c 3 ≦ 250 I I I d 5 ≦ 350 I I I e 10 ≦ 550 I I II In Table 1 above, character I denotes "good", character III denotes "bad" and character II denotes "slightly bad". It will be seen from Table 1 that when the rich layer 62 is formed to a thickness of 100-500 Å, external appearance and molding property of the molded lens can be remarkably improved. The rich layer (silicon rich layer) 62 having the high melting point exhibits an effect of suppressing production of minute foaming even if there is large temperature difference at the contact surface between the die and the glass blank. Furthermore, since evaporated constituents are scattered into the rich layer, the rich layer has an effect of delaying scatter of the evaporated constituents to the surface of the molded lens. This effect can be ascertained from the fact that the Auger electron spectral analyzer shows that surface of the molded lens contains such elements as boric acid, barium, etc. In order to improve external appearance of the molded lens, chemical processing (washing) for molding is quite effective for suppressing production of foaming even if sharp temperature difference takes place between the molding die and the glass blank. Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the claims, they should be construed as being included therein.

Claims: A machine (K) for molding an optical element by using a molding block in which upper and lower molding dies (17, 18), a sleeve die (19) for supporting the molding dies (17, 18) and a glass blank (20) are integrally combined with each other, comprising: a chamber (1) which accommodates a heating zone (3-5, 10-12) for heating the molding block from above and below, a deformation zone (6, 13) for pressing the molding dies (17, 18) of the molding block and a cooling zone (7-9, 14-16) for cooling the molding block and is formed with an inlet (26) for loading the molding block into the chamber (1) and an outlet (29) for discharging the molding block from the chamber (1); the heating zone including a plurality of sections (3-5, 10-12), while the cooling zone includes a plurality of sections (7-9, 14-16) such that the sections (3-5, 10-12) of the heating zone, the deformation zone (6, 13) and the sections (7-9, 14-16) of the cooling zone are controllable independently of one another; a transport means (34) for sequentially transporting the molding block through the heating zone (3-5, 10-12), the deformation zone (6, 13) and the cooling zone (7-9, 14-16); first and second shutters (28, 30) for opening and closing the inlet (26) and the outlet (29), respectively; and a gas supply means (25) for introducing nonoxidizing gas into the chamber (1). A machine (K) as claimed in Claim 1, wherein each of the sections (3-5, 10-12) of the heating zone, the deformation zone (6, 13) and the sections (7-9, 14-16) of the cooling zone is constituted by upper and lower heating plates (3-9, 10-16) interposing the molding dies (17, 18) therebetween and each of the upper and lower heating plates (3-9, 10-16) is provided with a temperature control means (22) based on thermal characteristics of the glass blank (20) gripped between the molding dies (17, 18) and a pressure control means (21). A machine (K) as claimed in Claim 1, wherein the number of steps in each of the heating zone, the deformation zone and the cooling zone can be changed arbitrarily. A machine (K) as claimed in Claim 1, wherein the transport means (34) is constituted by two cylinders (35, 36) and a pusher (37) having a comb-like shape. A machine (K) as claimed in Claim 1, wherein each of the sections (3-5, 10-12) of the heating zone, the deformation zone (6, 13) and the sections (7-9, 14-16) of the cooling zone includes a vertically movable member (21), wherein a lead wire (43) for a heater (22), a thermocouple (44) and a cooling pipe (47, 48) are provided in the vicinity of the vertically movable member (21) so as to be secured to a portion of the vertically movable member (21) in the chamber (1) such that fracture of the lead wire (43), the thermocouple (44) and the cooling pipe (47, 48) due to bending stress produced at the time of vertical travel of the vertically movable member (21) is prevented. A machine (K) as claimed in Claim 1, wherein each of the sections (3-5, 10-12) of the heating zone, the deformation zone (6, 13) and the sections (7-9, 14-16) of the cooling zone is constituted by upper and lower heating plates (3-9, 10-16) such that at least one of the upper heating plates (10-16) is provided with a release means (57, 58) for releasing the one of the upper heating plates (10-16) from the upper die (17). A machine (K) as claimed in Claim 6, wherein the release means (57, 58) includes a pin (57) movably embedded in the one of the upper heating plates (10-16) and a compression spring (58) for urging the pin (57) towards the upper die (17). A machine (K) as claimed in Claim 1, wherein when the molding block is loaded into and discharged from the chamber (1), either one of the first and second shutters (28, 30) is in a closed state. A machine (K) as claimed in Claim 1, wherein temperature control and pressure control in the heating zone (3-5, 10-12), the deformation zone (6, 13) and the cooling zone (7-9, 14-16) are performed in 7 steps. A machine (K) as claimed in Claim 1, wherein each of the sections (3-5, 10-12) of the heating zone, the deformation zone (6, 13) and the sections (7-9, 14-16) of the cooling zone is constituted by upper and lower heating plates (3-9, 10-16) such that each of the upper and lower heating plates (3-9, 10-16) is protected, except for its face held in contact with a corresponding one of the upper and lower dies (17, 18), by a heat insulating plate (41a-41e, 50a-50e). A machine (K) as claimed in Claim 10, wherein the heat insulating plate (41a-41e, 50a-50e) is formed by a vitrified wheel mainly consisting of aluminum oxide and silicon carbide. A machine (K) as claimed in Claim 1, wherein each of the sections (3-5, 10-12) of the heating zone, the deformation zone (6, 13) and the sections (7-9, 14-16) of the cooling zone is constituted by upper and lower heating plates (3-9, 10-16) and each of the upper and lower heating plates (3-9, 10-16) is provided with a heater (22) and a gap regulating member (54) for filling a gap between the heater (22) and each of the upper and lower heating plates (3-9, 10-16). A machine (K) as claimed in Claim 12, wherein the gap regulating member is formed by polishing abrasive grains (54) made of aluminum oxide, silicon carbide and zirconium oxide. A machine as claimed in claim 11, wherein the heat insulation plate (41a-41e; 50a-50e) has a porosity of 40% or less. A method of producing an optical element by a machine (K) as claimed in anyone of the claims 1 - 14, comprising: the first step of integrally combining upper and lower molding dies (17, 18), a sleeve die (19) for supporting the molding dies (17, 18) and a glass blank (20) with each other so as to obtain a molding block; the second step of heating the molding block from above and below in one or more steps. the third step of pressing the glass blank (20) through the molding dies (17, 18) in one or more steps. so as to deform the glass blank (20); and the fourth step of cooling the molding block from above and below in one or more steps. the second to fourth steps being continuously performed in nonoxidizing atmosphere in an identical chamber (1). A method as claimed in Claim 15, wherein the second to fourth steps are performed in a total of 7 sections. A method as claimed in Claim 15, wherein in the fourth step, temperature difference is indirectly provided between the upper and lower dies (17, 18) in accordance with shape of the optical element. A method as claimed in Claim 17, wherein in order to provide temperature difference between the upper and lower dies (17, 18), an upper heating plate (15) is held out of contact with the upper die (17). A method as claimed in Claim 15, wherein in the fourth step, cooling of the molding block from the deformation temperature of the glass blank (20) to a transition point of glass is performed once or a plurality of times. A method as claimed in Claim 15, wherein a molding face of the glass blank (20) is confined in an optically functional face (D) of each of the upper and lower dies (17, 18). A method as claimed in Claim 20, wherein a face of the molded optical element other than the molding face is processed by a mechanical means. A method as claimed in any of the claims 15 - 21, in which a material for a molded glass lens is used having an excessive layer with a high melting point formed on a surface of a lens blank (63) made of glass. A method as claimed in any one of Claims 15 to 19, wherein constituents having low melting points and contained in glass are selectively eluted from a surface of glass by a chemical technique such that an excessive layer (62) of compositions having high melting points is formed on the surface of glass, whereby the glass blank (20) is obtained.

Inventors: INOUE TAKASI, NAKAMURA SHOJI, SHIOYAMA TADAO, SUNOHARA MASAAKI, INOUE, TAKASI, NAKAMURA, SHOJI, SHIOYAMA, TADAO, SUNOHARA, MASAAKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1991
Main classification: C03B11
IPC codes: C03B11