Production method for optical element

Abstract

Provided is a production method that makes it possible to efficiently produce a high-precision optical element by means of a simple device. According to the present invention, a production method for an optical element involves placing a core member inside a cavity that is enclosed by surfaces that include a surface C of a first section of a mold and a surface D of a second section and, while a layer is formed on a surface A of the core member by injection molding, pressing a surface B that is on the opposite side from the surface A onto the surface D as heated to a temperature that is higher than the glass transition temperature of a plastic of the core member and then cooling the surface D to make the surface B a surface Bd that has a target shape. The shape of the core member is determined such that the ratio of the volume of a gap that is formed between the surface B and the surface D when the core member has been placed inside the cavity such that only a portion of the surface B is in contact with the surface D and the volume of a space is no more than 0.15 when the core member has been placed inside the cavity such that only a portion of the surface B contacts the surface D.

Description

Optical element manufacturing method

[0001] The present invention relates to a method for manufacturing optical elements, including lenses, diffraction gratings, prisms, and microlens arrays.

[0002] In the manufacturing method of plastic injection molding, the molten plastic injected into the mold must be sufficiently cooled before the molded product can be removed, so production efficiency depends on the cooling time. If the molded product is thick, the cooling time will be longer, which will reduce production efficiency. In the case of optical elements, the mold temperature must be relatively high to maintain high shape precision, so the cooling time has a particularly large impact on production efficiency.

[0003] For this reason, a manufacturing method for optical elements by injection molding has been developed in which molding is divided into multiple steps, a core member is manufactured in the first molding step, and a thin layer is formed on the surface of the core member in the second and subsequent molding steps (for example, Patent Document 1). According to this method, the optical surface that requires high shape accuracy is formed as the surface of a thin layer in a separate molding step, thereby shortening the cooling time and improving production efficiency.

[0004] However, in the above manufacturing method, it is usually necessary to separately perform the molding steps of thin layers on both sides of the core member while the core member is fixed in a mold, which requires a complex mechanism of the mold and auxiliary equipment.

[0005] As described above, a manufacturing method capable of efficiently manufacturing high-precision optical elements using simple equipment has not been developed. Therefore, there is a need for a manufacturing method capable of efficiently manufacturing high-precision optical elements using simple equipment.

[0006] JPH09225961(A) (Patent Publication No. 9-225961)

[0007] A technical object of the present invention is to provide a manufacturing method that can efficiently manufacture high-precision optical elements using a simple apparatus.

[0008] A method for manufacturing an optical element of the present invention uses a mold having a first portion and a second portion. In the manufacturing method of the present invention, a core member is placed in a cavity of the mold surrounded by surfaces including surface C of the first portion and surface D of the second portion opposite surface C, and molten plastic is poured into the space surrounded by surfaces including surfaces A and C of the core member to form a layer on surface A by injection molding. Meanwhile, surface B of the core member opposite surface A is pressed against surface D, which has been heated to a temperature higher than the glass transition temperature of the plastic of the core member, and surface D is then cooled to mold surface B into a surface Bd of a desired shape. In the manufacturing method of the present invention, the core member is placed in the cavity so that only a portion of surface B is in contact with surface D, leaving a gap between surfaces B and D, and the shape of the core member is determined so that the ratio of the volume of the gap to the volume of the space is 0.15 or less.

[0009] In the method for manufacturing an optical element of the present invention, a layer is formed on surface A by injection molding, while surface B of the core member opposite surface A is pressed against surface D, which has been heated to a temperature higher than the glass transition temperature of the plastic of the core member, thereby press-molding surface B into surface Bd of the desired shape, thereby allowing high-precision optical elements to be obtained with simple equipment.

[0010] Furthermore, the manufacturing method of the present invention can improve the surface accuracy of the optical surface of the optical element corresponding to surface B.

[0011] In the method for manufacturing an optical element according to the first embodiment of the present invention, the maximum distance between the surface B and the surface D in the thickness direction of the optical element is ⅓ or less of the maximum thickness of the optical element.

[0012] In the method for manufacturing an optical element according to the second embodiment of the present invention, the maximum thickness of the optical element is 5 millimeters or more.

[0013] The method of the present invention is particularly effective for producing optical elements having a center thickness or maximum thickness of 5 mm or more, in which deformation of surface B of the core member is unlikely to occur due to the filling pressure of the molten plastic alone.

[0014] In the method for manufacturing an optical element according to the third embodiment of the present invention, the ratio of the volume of the gap to the volume of the space is 0.015 or more and 0.08 or less.

[0015] According to this embodiment, the pressure after the expansion of the plastic, that is, the pressure at the end of molding of surface B, can be set to a sufficient value, thereby further improving the precision of the optical surface of the optical element corresponding to surface B.

[0016] In the method for manufacturing an optical element according to the fourth embodiment of the present invention, the surface A is a plane perpendicular to the central axis of the core member.

[0017] In the method for manufacturing an optical element according to the fifth embodiment of the present invention, the surface Bd is a convex surface or a concave surface.

[0018] In the method for manufacturing an optical element according to the sixth embodiment of the present invention, the surface Bd and the surface B are symmetrical about their respective central axes.

[0019] In the method for manufacturing an optical element according to the seventh embodiment of the present invention, the surface Bd is an aspherical surface.

[0020] In the method for manufacturing an optical element according to the eighth embodiment of the present invention, the material of the core member and the material of the layer formed on surface A by injection molding are the same.

[0021] In the method for manufacturing an optical element according to the ninth embodiment of the present invention, the filling pressure is 50 megapascals or more.

[0022] If the filling pressure is less than 50 megapascals, a sufficient pressure value cannot be secured at the end of molding of surface B, and therefore the precision of the optical surface cannot be further improved.

[0023] 6A . FIG. 6B is a diagram showing an example of a mold used in the manufacturing method of an optical element of the present invention. FIG. 6C is a flow chart explaining the manufacturing method of an optical element of the present invention. FIG. 6D is a diagram showing a core member placed in a cavity between a first part and a second part of a mold. FIG. 6E is a diagram showing a state in which molten plastic has been poured into the space between surface C of the first part and surface A of the core member. FIG. 6F is a diagram showing a state after press molding has been performed. FIG. 6G is a perspective view of an example of a core member. FIG. 6H is a longitudinal cross-sectional view of the core member of FIG. 6H. FIG. 6H is a transverse cross-sectional view of the core member of FIG. 6H. FIG. 6I is a perspective view of an example of an optical element. FIG. 7A is a longitudinal cross-sectional view of the optical element of FIG. 7A. FIG. 7A is a transverse cross-sectional view of the optical element of FIG. 7I. FIG. 6I is a diagram showing a core member arranged in a mold. FIG. 6I is a diagram showing the relationship between pressure, volume, and temperature of plastic (PMMA, polymethyl methacrylate). FIG. 6J is a diagram showing the relationship between the temperature and pressure of plastic and the theoretical value of the ratio of volume Vg to volume V, assuming that volume Vg is filled by the volume expansion of the plastic. FIG. 6I is a flow chart explaining the manufacturing method of an optical element of the prior art. FIG. 6H is a perspective view of a core member as a primary molded product. FIG. 6I is a longitudinal cross-sectional view of a core member as a primary molded product. FIG. 6I is a transverse cross-sectional view of a core member as a primary molded product. 1 is a perspective view of a secondary molded product; a longitudinal sectional view of the secondary molded product; a transverse sectional view of the secondary molded product; a perspective view of a tertiary molded product; a longitudinal sectional view of the tertiary molded product; a transverse sectional view of the tertiary molded product; a view showing a core member placed in a cavity between the first and second parts of a mold when manufacturing a concave lens; a view showing a state in which molten plastic is poured into a space between surface C of the first part and surface A of the core member when manufacturing a concave lens; a view showing a state after press molding is performed when manufacturing a concave lens; a view showing a core member placed in a cavity between the first and second parts of a mold when manufacturing an optical element such as a prism in which one surface is inclined relative to the other surface; a view showing a state in which molten plastic is poured into a space between surface C of the first part and surface A of the core member when manufacturing an optical element such as a prism in which the other surface is inclined relative to the one surface; a view showing a state after press molding is performed when manufacturing an optical element such as a prism in which the other surface is inclined relative to the one surface.

[0024] FIG. 1 shows an example of a mold used in the method for manufacturing an optical element of the present invention. The mold includes a first portion 110 and a second portion 120 that face each other and form a cavity therebetween. The second portion 120 includes a press-molding portion 121 equipped with a heater 123 and an outer portion 127 surrounding the press-molding portion 121. The press-molding portion 121 and the outer portion 127 are separated by a heat insulating material 125. The heater 123 may be a commercially available electric heater. The heat insulating material 125 may be a sheet-like material made of, for example, glass cloth. The press-molding portion 121 and the heat insulating material 125 are configured to be stored in the outer portion 127.

[0025] FIG. 2 is a flow chart illustrating the method for manufacturing an optical element according to the present invention.

[0026] 2, the core member 210 is molded. The core member 210 is made of plastic and may be molded by injection molding, for example.

[0027] In step S1020 of FIG. 2, the core member 210 is placed in the cavity of the mold shown in FIG.

[0028] 3 is a diagram showing a core member 210 installed in a cavity between the first portion 110 and the second portion 120 of a mold. The core member 210 has a surface A and an opposite surface B. A space 115 is formed between surface C of the first portion 110 and surface A of the core member 210. In the state shown in FIG. 3, the apex of surface B of the core member 210 and the portion (valley bottom) corresponding to the apex of surface D of the second portion 120 are in contact.

[0029] 2, molten plastic is poured into the space 115 between surfaces C and A to form a layer on surface A of the core member 210 by injection molding, while the injection molding filling pressure is applied to the core member 210 through surface A of the core member 210, thereby pressing surface B, the side opposite surface A of the core member 210, against surface D of the press-molding portion 121, which has been heated to a temperature higher than the glass transition temperature of the plastic of the core member 210, to perform press molding. The shape of surface D of the mold is transferred to surface B of the core member by press molding, forming surface B'. The side surfaces of the core member 210 other than surfaces A and B are in contact with the inner surface of the second portion in a manner that allows the core member 210 to move inside the second portion.

[0030] 4 is a diagram showing a state in which a portion of molten plastic has been poured into the space 115 between surface C of the first portion 110 and surface A of the core member 210. The poured molten plastic forms a layer on surface A of the core member 210. Thereafter, the filling pressure of the plastic poured into the space between surface C and surface A applies pressure to the core member 210 through surface A of the core member 210, and surface B of the core member 210 opposite surface A is pressed against surface D of a mold that has been heated by heater 123 to a temperature higher than the glass transition temperature of the plastic of the core member 210, thereby performing press molding.

[0031] 5 is a diagram showing the state after press molding has been performed, in which the shape of surface D of the mold is transferred to the core member through press molding, forming surface B'.

[0032] Table 1 shows an example of conditions for the method of manufacturing an optical element of the present invention. Since the heating temperature of the forming heater is set to 150° C., the temperature of surface D is higher than the glass transition temperature of the material of the core member, and the shape is transferred with high precision.

[0033] In the above-described embodiment, the core material and the layer formed by injection molding are the same material. In general, the core material may be different from the layer formed by injection molding. Specifically, for example, when the layer formed by injection molding is made of PMMA, the core material may be made of PC (polycarbonate), which has a relatively high glass transition temperature.

[0034] In step S1040 of FIG. 2, heating by the heater 123 is stopped, and the surface D of the press-molding portion 121 and the molded product are cooled to a temperature below the glass transition temperature of the material.

[0035] The molded product is removed from the mold in step S1050 of Fig. 2. After the molded product is removed from the mold, heating by heater 123 begins for the next molding process.

[0036] Normally, the above-mentioned cooling time of the press-molding part 121 and the molded product does not affect the molding time because it is within the time required to solidify the molten plastic of the layer formed by injection molding on surface A. If necessary, the above-mentioned cooling time can be shortened by providing a water-cooling pipe to the press-molding part 121.

[0037] 6A is a perspective view of an example of a core member 210. The length and width of the bottom surface of the core member 210 are 62 millimeters and 40 millimeters, respectively, and the height of the core member is 18 millimeters.

[0038] FIG. 6B is a longitudinal cross-sectional view of the core member of FIG. 6A.

[0039] FIG. 6C is a cross-sectional view of the core member of FIG. 6A.

[0040] 7A is a perspective view of an example of an optical element 300. The optical element 300 comprises a portion 215 corresponding to a core member after press molding and an additional portion 220 formed as a layer by injection molding. The length, width, and height (thickness) of the additional portion 220 are 75 millimeters, 50 millimeters, and 2.5 millimeters, respectively. The height of the optical element 300 is 20.5 millimeters.

[0041] FIG. 7B is a longitudinal cross-sectional view of the optical element of FIG. 7A.

[0042] FIG. 7C is a cross-sectional view of the optical element of FIG. 7A.

[0043] In this example, surface B' is a lens surface. The shape of surface Bd, which is the target shape of the lens surface, can be expressed by the following formula (1): The unit of length is millimeters. z represents the coordinate in the direction of the central axis based on the vertex of the lens surface, r represents the distance from the central axis to a point on the surface, and A1-A4 represent coefficients.

[0044] The shape of the surface Bd, which is the target shape of the lens surface, is symmetrical about the central axis. Next, the shape of the core member 210 will be described.

[0045] 8 is a diagram showing the core member 210 placed in a mold, which shows the same state as that shown in FIG.

[0046] The contour of surface D in Figure 8 is the same as the contour of surface Bd. The shape of surface B of the core member 210 corresponding to surface Bd is also symmetrical around the central axis. As described with reference to Figure 3, the vertex of surface B of the core member 210 and the portion (valley bottom) of the second portion 120 corresponding to the vertex of surface D are in contact. Because the contours of surface D and surface B are different, a gap 117 is created between surface D and surface B. In the example shown in Figure 8, if coordinates in a plane perpendicular to the central axis are represented by (x, y), the shape of surface B is defined so that the difference g in coordinates in the central axis direction between points on surface D and surface B with the same (x, y) coordinates increases with distance from the central axis.

[0047] The shape of the surface Bd and the shape of the surface B of the core member 210 are axially symmetric, but the shapes of the core member 200 and the lens 300 differ in length in the vertical direction and in the horizontal direction, as shown in Figures 6A to 6C and 7A to 7C, respectively. The value of g reaches a maximum value gmax at the end of the core member 200 in Figure 8, which shows the vertical cross section of the core member 200.

[0048] The difference between the center thickness of the molded lens and the center thickness of the core member 210 is t. The volume of the space 115 between surface C of the first portion 110 and surface A of the core member 210, into which the plastic is poured, is represented by V, and the volume of the gap 117 between surface B of the core member 210 and surface D of the second portion 120 is represented by Vg.

[0049] The width e of deviation from the design value for surface B of the core member and surface B' of the optical element 300 will be explained. Surface B' is a lens surface. Surface Bd and the surface of the measured shape (hereinafter referred to as the measurement surface) are superimposed with their vertices and central axes aligned. An xyz Cartesian coordinate system is defined with the vertex of the lens surface as the origin and the central axis of the lens surface as the z-axis. The shape of surface Bd is expressed by equation (1). r in equation (1) can be expressed by the following equation: Let P be a point on the surface Bd, and find the value obtained by subtracting the z coordinate of point P from the z coordinate of a point on the measurement surface that has the same (x, y) coordinate as point P. Find the above value for each point on the surface Bd. In the set of the above values ​​for points on the surface Bd, the difference between the maximum and minimum values ​​is defined as the deviation width e from the design value.

[0050] The deviation width e of surface B of core member 210 shown in Figure 6 with respect to the design surface is 595 micrometers. The deviation width e of surface B' of optical element 300 manufactured by the present invention shown in Figure 7 is 75 micrometers. The deviation width of surface B' is significantly reduced compared to the deviation width of surface B.

[0051] It is thought that when filling pressure is applied to surface A of the core member, surface B of the core member, which is in contact with surface D, which has a temperature higher than the glass transition temperature, remelts, causing deformation and flow on surface B and reducing the range of deviation.

[0052] The inventors of the present application have newly discovered that, in order to manufacture high-precision optical elements using the above-described method, it is important to set an appropriate value for the ratio of the volume Vg of the gap 117 between surfaces B and D to the volume V of the space 115 between surfaces C and A, into which the molten plastic is poured, when the core member 210 is placed in the cavity with part of surface B of the core member 210 in contact with surface D of the second portion 120 (the state shown in FIG. 8 ). This discovery is described below.

[0053] When the core member is relatively thick and has relatively high rigidity, deformation of surface B of the core member is not caused solely by the filling pressure of the molten plastic poured into the space between surfaces C and A. In this case, deformation of surface B occurs as follows: When the plastic poured into the space between surfaces C and A expands and applies pressure to the core member, surface B of the core member is pressed against surface D of second portion 120, which has been heated to a temperature higher than the glass transition temperature of the plastic of core member 210, causing deformation and flow of surface B. Gap 117 between surfaces B and D shown in FIG. 8 is filled by the core member, and surface A of the core member moves toward the second portion. At this time, the plastic in the space between surfaces C and A expands from the state shown in FIG. 4 to the state shown in FIG. 5.

[0054] Figure 9A shows the relationship between pressure, volume, and temperature for plastic (PMMA, polymethyl methacrylate). The horizontal axis of Figure 9A represents temperature, and the vertical axis of Figure 9A represents specific volume. At a temperature of 250°C, the specific volume at a filling pressure of 200 MPa is 0.853, the specific volume at a filling pressure of 100 MPa is 0.886, the specific volume at a filling pressure of 50 MPa is 0.910, and the specific volume at a filling pressure of 0 MPa is 0.942. Therefore, the volume expansion rate when pressure is completely released is as follows: 200 MPa: 1.104 100 MPa: 1.062 50 MPa: 1.036 As described above, the plastic poured into the space between surfaces C and A expands, exerting pressure on the core member, causing surface B of the core member in contact with surface D, which has been heated to a temperature higher than the glass transition temperature, to remelt. Deformation and flow occur at surface B, filling volume Vg with the core member, and surface A of the core member moving toward surface B. In other words, volume Vg is equal to the expansion of the plastic in volume V of space 115 between surfaces C and A in the state shown in Figure 8. Therefore, the ratio Vg / V of volume Vg to volume V is related to the above-mentioned volumetric expansion rate of core member 200. In the above process, the pressure decreases as the plastic expands.

[0055] 9B is a diagram showing the relationship between the temperature and pressure of the plastic and the theoretical value of the ratio of volume Vg to volume V, assuming that volume Vg is equal to the expansion of the plastic in volume V of space 115 between surface C and surface A in the state of FIG. 8. The horizontal axis of FIG. 9B represents temperature, and the vertical axis of FIG. 9B represents the theoretical value of the ratio of volume Vg to volume V. When the temperature is 250°C, the theoretical value of the ratio Vg / V of volume Vg to volume V is as follows: 200 MPa: 0.104 100 MPa: 0.062 50 MPa: 0.036

[0056] Considering the types of plastic materials that have large volume changes, the upper limit of the ratio Vg / V is 0.15.

[0057] In the above-mentioned example, the conditions for the ratio Vg / V of the volume Vg to the volume V are as follows: Molding temperature: 250°C Filling pressure: 70 MPa Expansion coefficient: 1.046 (when the pressure is reduced from 70 MPa to 0 MPa) Vg: 9375 mm 3 (cubic millimeters) V: 210mm 3 (cubic millimeters) Vg / V: 0.0224 When Vg / V is the above value, the pressure after the plastic expands will not drop to 0. Generally, to obtain sufficient shape precision for the optical surface of surface B', it is preferable that the pressure after the plastic expands, i.e., the pressure at the end of molding surface B, be 30 megapascals or higher. Also, to obtain an optical surface with sufficient shape precision, it is preferable that Vg / V be 0.015 or higher.

[0058] The method of the present invention is particularly effective for manufacturing optical elements with a center or maximum thickness of 5 millimeters or more, where deformation of surface B of the core member is unlikely to occur due to the filling pressure of the molten plastic alone. The maximum value gmax of the gap g in the thickness direction of the optical element, i.e., the direction in which surface A of the core member moves when pressed, is 1 / 3 or less of the center thickness (maximum thickness) of the optical element.

[0059] In the above-described embodiment, the center thickness of the optical element and the maximum value gmax of the gap g are as follows: Center thickness: 20.5 mm gmax: 0.15 mm

[0060] Here, a conventional method for manufacturing an optical element will be described.

[0061] FIG. 10 is a flow chart illustrating a prior art method for manufacturing an optical element.

[0062] In step S2010 of FIG. 10, a core member 210' is manufactured as a primary molded product.

[0063] FIG. 11A is a perspective view of a core member 210' as a primary molded product.

[0064] FIG. 11B is a vertical cross-sectional view of a core member 210' as a primary molded product.

[0065] FIG. 11C is a cross-sectional view of the core member 210' as a primary molded product.

[0066] The length and width of the bottom surface of the primary molded product are 57 mm and 35 mm, respectively, and the height of the primary molded product is 15.5 mm.

[0067] 10, a core member 210' as a primary molded product is placed in a mold, and an additional portion 220' is formed on the bottom surface of the core member 210' to manufacture a secondary molded product. An example of the mold used is the mold shown in FIG. 1 of Patent Document 1.

[0068] FIG. 12A is a perspective view of the secondary molded product.

[0069] FIG. 12B is a vertical cross-sectional view of the secondary molded product.

[0070] FIG. 12C is a cross-sectional view of the secondary molded product.

[0071] The length, width and height (thickness) of the additional portion 220' are 75 mm, 50 mm and 2.5 mm, respectively, and the height of the secondary molded product is 18 mm.

[0072] In step S2030 of Figure 10, with the secondary molded product placed in the above-mentioned mold, an outer layer 230' is formed on the surface of the core member 210' opposite to the bottom surface on which the additional portion 220' is formed, thereby producing a tertiary molded product.

[0073] FIG. 13A is a perspective view of the tertiary molded product.

[0074] FIG. 13B is a vertical cross-sectional view of the tertiary molded product.

[0075] FIG. 13C is a cross-sectional view of the tertiary molded product.

[0076] The length and width of the bottom surface of the portion of the third molded article covered with the outer layer 230' are 62 mm and 40 mm, respectively. The thickness of the outer layer 230' is 2.5 mm, and the height of the portion of the third molded article covered with the outer layer 230' is 18 mm from the surface of the additional portion 220'. The height of the third molded article is 20.5 mm.

[0077] The width of deviation e of surface B" of optical element 300' manufactured by the conventional manufacturing method shown in FIG. 13A is 104 micrometers. Therefore, when forming an aspherical lens surface as expressed in formula (1), the width of deviation e of surface B' of optical element 300 manufactured by the manufacturing method of the present invention is 75 micrometers as described above, which is smaller than the width of deviation e of surface B" of optical element 300' manufactured by the conventional manufacturing method.

[0078] 14 to 16 are diagrams showing a core member and a mold for manufacturing a concave lens.

[0079] 14 is a diagram showing a core member 210 installed in a cavity between the first portion 110 and the second portion 120 of the mold, similar to FIG. 3. The core member 210 has a surface A and an opposite surface B. A space 115 is formed between surface C of the first portion 110 and surface A of the core member 210.

[0080] The shape of surface D in Figure 14 is the same as the shape of surface Bd, which is the target shape of the lens surface. The shape of surface B of the core member 210, which corresponds to surface Bd, which is symmetrical about the central axis, is also symmetrical about the central axis. In the state shown in Figure 14, the vertex of surface B of the core member 210 and the part of the second portion 120 corresponding to the vertex of surface D are in contact. Because the contours of surface D and surface B are different, a gap occurs between surface D and surface B. If coordinates in a plane perpendicular to the central axis are represented as (x, y), the difference g in the coordinates in the central axis direction between a point on surface D and a point on surface B with the same (x, y) coordinates is set to increase with the distance from the central axis. The value of g reaches a maximum value gmax at the end of the core member 200 in Figure 15, which shows a longitudinal cross section of the core member 200.

[0081] 15 , like FIG. 4 , is a diagram showing a state in which a portion of molten plastic has been poured into the space 115 between surface C of the first portion 110 and surface A of the core member 210. This diagram shows a state in which a portion of molten plastic has been poured into the space 115 between surface C of the first portion 110 and surface A of the core member 210. The poured molten plastic forms a layer on surface A of the core member 210. When the plastic poured into the space between surfaces C and A expands and applies pressure to the core member 210, surface B of the core member 210 is pressed against surface D of the second portion 120, which has been heated to a temperature higher than the glass transition temperature of the plastic of the core member 210. This causes deformation and flow in surface B, filling the gap between surfaces B and D with the core member 210, and surface A of the core member 210 moving toward the second portion. In this manner, surface B is press-molded.

[0082] Fig. 16 is a diagram showing the state after press molding has been carried out, similar to Fig. 5. Surface B' has been formed by transferring the shape of surface D of the mold to the core member through press molding.

[0083] 17 to 19 are diagrams showing a core member and a mold when manufacturing an optical element such as a prism in which one surface is inclined relative to the other surface.

[0084] 17 is a diagram showing a core member 210 installed in a cavity between the first portion 110 and the second portion 120 of a mold, similar to FIG. 3 . The core member 210 has a surface A and an opposite surface B. A space 115 is formed between surface C of the first portion 110 and surface A of the core member 210. In the state shown in FIG. 17 , the end of surface B of the core member 210 and the end of surface D of the second portion 120 are in contact. Since surface D is inclined with respect to surface B, a gap is created between the two surfaces. The gap (the distance between surface B and surface D) in the direction in which surface A of the core member moves when pressed increases with the distance from the portion where surface B and surface D are in contact.

[0085] 18 , like FIG. 4 , is a diagram showing a state in which a portion of molten plastic has been poured into the space 115 between surface C of the first portion 110 and surface A of the core member 210. This diagram shows a state in which a portion of molten plastic has been poured into the space 115 between surface C of the first portion 110 and surface A of the core member 210. The poured molten plastic forms a layer on surface A of the core member 210. When the plastic poured into the space between surfaces C and A expands and applies pressure to the core member 210, surface B of the core member 210 is pressed against surface D of the second portion 120, which has been heated to a temperature higher than the glass transition temperature of the plastic of the core member 210. This causes deformation and flow in surface B, filling the gap between surfaces B and D with the core member 210, and surface A of the core member 210 moving toward the second portion. In this manner, surface B is press-molded.

[0086] Fig. 19 is a diagram showing the state after press molding has been carried out, similar to Fig. 5. Surface B' has been formed by transferring the shape of surface D of the mold to the core member through press molding.

[0087] The apparatus used in the manufacturing method of the present invention is, as shown in FIG. 1, merely a simple heater added to a normal mold, and does not require a press mechanism or the like.

[0088] On the other hand, the apparatus used in the conventional manufacturing method includes a complex mechanism for performing two molding steps to form thin layers on both sides of the core member while the core member is fixed in a mold, as shown in FIG. 1 of Patent Document 1.

[0089] When the apparatus used in the manufacturing method of the present invention shown in FIG. 1 is compared with the apparatus shown in FIG. 1 of Patent Document 1, the former is much simpler.

[0090] Therefore, according to the manufacturing method of the present invention, optical elements with high precision equal to or higher than that of the prior art can be efficiently manufactured using a simpler apparatus than that of the prior art.

Claims

1. A method for manufacturing an optical element using a mold having a first portion and a second portion, wherein a core member is placed within a cavity of the mold surrounded by surfaces including surface C of the first portion and surface D of the second portion opposite surface C, and molten plastic is poured into the space surrounded by surfaces including surfaces A and C of the core member to form a layer on surface A by injection molding, while surface B of the core member opposite surface A is pressed against surface D, which has been heated to a temperature higher than the glass transition temperature of the plastic of the core member, and surface D is then cooled to mold surface B into a target shape of surface Bd, and the core member is placed within the cavity so that only a portion of surface B is in contact with surface D and a gap exists between surfaces B and D, and the shape of the core member is determined so that the ratio of the volume of the gap to the volume of the space is 0.15 or less.

2. The method for manufacturing an optical element according to claim 1, wherein the maximum distance between said surface B and said surface D in the thickness direction of said optical element is 1 / 3 or less of the maximum thickness of said optical element.

3. The method for producing an optical element according to claim 1, wherein the maximum thickness of the optical element is 5 millimeters or more.

4. The method for manufacturing an optical element according to claim 1, wherein the ratio of the volume of the gap to the volume of the space is 0.015 or more and 0.08 or less.

5. The method for manufacturing an optical element according to claim 1, wherein the surface A is a plane perpendicular to the central axis of the core member.

6. The method for manufacturing an optical element according to claim 1, wherein the surface Bd is a convex or concave surface.

7. The method for manufacturing an optical element according to claim 1, wherein the surface Bd and the surface B are symmetrical about their respective central axes.

8. The method for manufacturing an optical element according to claim 1, wherein the surface Bd is an aspherical surface.

9. The method for manufacturing an optical element according to claim 1, wherein the material of the core member is the same as the material of the layer formed on surface A by injection molding.

10. The method for manufacturing an optical element according to claim 1, wherein the injection molding filling pressure is 50 megapascals or more.

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