Patent Publication Number: US-2013249128-A1

Title: Injection mold and method for molding an optical element

Description:
This application is based on Japanese Patent Application No. 2004-191837 filed on Jun. 29, 2004, the content of which is herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an injection mold and a method for molding an optical element, and more particularly to an injection mold for molding a small and light optical element, such as a lens, an optical waveguide, etc., and a method for molding an optical element. 
     2. Description of Related Art 
     In recent years, with improvement of resin materials and injection molding techniques, various kinds of small and light lenses, prism plates and optical waveguides have been developed, and a demand for use of these optical elements for optical pick-up devices and portable telephones has been stronger. In order to produce such optical elements, molds which permit accurate transfer of fine configurations for diffraction, fine configurations for prism surfaces, blaze surfaces, etc. and smooth surfaces are required. 
     In order to achieve high accuracy transfer, Japanese Patent Laid-Open Publication No. 2002-96335 suggests a mold 50 as shown by  FIG. 7 . The mold 50 has a heat insulating layer 53 between a core base 52 located in the center of a mold base 51 and a surface processed layer 54. A cavity 60 is formed between the surface processed layer 54 with a blaze surface 54a, which is of a fine configuration, and a mold base 55. The heat insulating layer 53 is preferably a ceramic flame coating, and the surface processed layer 54 is preferably a nickel plating. 
     Since the mold 50 has a heat insulating layer 53 in the rear of the fine configuration (blaze surface 54a), the heat retaining property of the blaze surface 54a is improved, and it is possible to transfer the fine configuration to a molded product at high accuracy. However, at an area 51a next to the blaze surface 54a, the mold base 51, which has a relatively high coefficient of thermal conductivity, is exposed. Therefore, in this area 51a, heat radiation from melted resin injected into the cavity 60 is large, and it has been found that this influences the transfer accuracy of the fine configuration. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an injection mold and an optical element molding method which permit a further improvement in transfer accuracy of a fine configuration. 
     In order to achieve the object, a first aspect of the present invention provides an injection mold for molding an optical element out of resin comprising a heat insulating layer between a core base and a surface processed layer, wherein a mold base forming a cavity to be filled with resin comprises at least a part made of a heat insulating material, the part being adjacent to the surface processed layer. 
     The second aspect of the present invention provides a method for injection molding an optical element out of resin by use of an injection mold comprising at least a movable mold and a fixed mold, wherein the injection mold comprises a heat insulating layer between a core base and a surface processed layer, and a mold base forming a cavity to be filled with resin comprises at least a part made of a heat insulating material, the part being adjacent to the surface processed layer. 
     According to the first and second aspects of the present invention, the mold base may be wholly made of a heat insulating material, or alternatively, a heat insulator may be provided between the mold base and the surface processed layer. The heat insulating material and the heat insulator are, for example, stainless steel, titanium alloy, nickel alloy, ceramic or heat resistance resin. 
     According to the first and second aspects of the present invention, since a heat insulating layer is provided between the core base and the surface processed layer, the temperature of resin injected into the cavity can be kept well, and the transfer accuracy of especially a fine configuration formed on the surface processed layer is improved. Further, since the part of a mold base which is adjacent to the surface processed layer is made of a heat insulating material, heat radiation from resin around the fine configuration is inhibited, and the transfer accuracy of the fine configuration is further improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This and other objects and features of the present invention will be apparent from the following description with reference to the accompanying drawings, in which: 
         FIG. 1  is a sectional view of a mold according to a first embodiment of the present invention; 
         FIG. 2  is a sectional view of a mold according to a second embodiment of the present invention; 
         FIG. 3  is a sectional view of a mold according to a third embodiment of the present invention; 
         FIG. 4  is a sectional view of a mold according to a fourth embodiment of the present invention; 
         FIG. 5  is a sectional view of a mold according to a fifth embodiment of the present invention; 
         FIG. 6  is a sectional view of a mold according to a sixth embodiment of the present invention; and 
         FIG. 7  is a sectional view of a conventional injection mold. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of an injection mold and an optical element molding method according to the present invention are hereinafter described with reference to the accompanying drawings. In the drawings showing the respective embodiments, the same parts/members are denoted by the same reference numbers, and repetitious descriptions are avoided. 
     First Embodiment 
     See FIG.  1   
       FIG. 1  shows a mold  1 A according to a first embodiment of the present invention. The mold  1 A comprises a movable mold  10  and a fixed mold  20 . The movable mold  10  comprises bases  11  and  12 , a heat insulating layer  13  and a surface processed layer  14 . The fixed mold  20  comprises a base  21 . 
     The surface processed layer  14  is finished in accordance with the configuration of an optical surface of a product (optical element), such as a lens, a mirror, a prism plate, an optical waveguide, etc., and a fine configuration  14   a , such as a diffraction grating, a prism surface, a blaze surface, etc., is formed. A cavity  30  is formed of the surface processed layer  14  and internal surfaces of the bases  21  and  11 . 
     The bases  12  and  21  are made of a material usually used for mold bases, such as metal, for example, carbon steel, stainless steel or the like. The coefficient of thermal conductivity of carbon steel is 50 W/mK, and the coefficient of thermal conductivity of martensite stainless steel is 27 W/mK. 
     On the other hand, the base  11  is made of a heat insulating material. Here, for the base  11 , various materials with lower coefficients of thermal conductivity than that of the material of the bases  12  and  21  are usable. For example, ferrite stainless steel (with a coefficient of thermal conductivity of 17 W/mK), austenitic stainless steel (with a coefficient of thermal conductivity of 13 W/mK), titanium alloy (Ti-6Al-4V with a coefficient of thermal conductivity of 7.5 W/mK), nickel alloy (inconel with a coefficient of thermal conductivity of 15 W/mK), etc. are usable. 
     The heat insulating layer  13  is, for example, of ceramic flame-coated on the core base  12 , an organic material (heat resistant polymer) such as polyimide resin, sintered ceramic, which has a low coefficient of thermal conductivity, titanium alloy (Ti-6Al-4V, Ti-3Al-2.5V, Ti-6Al-7Nb, etc.), cermet (aluminum titanate, TiO 2 —Al 2 O 3 ), stainless steel (ferrite, austenitic, etc.), nickel alloy (inconel, FeNi), etc. The ceramic may be zirconia, silicon nitride, titanium nitride, etc. The surface processed layer  14  is a non-ferrous metal plating, such as a nickel plating, on the heat insulating layer  13 . 
     The heat insulating layer  13  is not necessarily made of one of the above materials, and can be made of any material as long as the material has a lower coefficient of thermal conductivity than that of the core base  12 . For example, a material with a coefficient of thermal conductivity which is, for example, lower than 20 W/mK can be used. 
     According to the first embodiment, since the heat insulating layer  13  exists between the core base  12  and the surface processed layer  14 , the temperature of resin injected into the cavity  30  is kept well. Thereby, the transfer accuracy of especially the fine configuration  14   a  formed on the surface processed layer  14  is improved. 
     The mold base  11 , which is a wall of the cavity  30 , has a part  11   a  adjacent to the surface processed layer  14 . Since the mold base  11  is wholly made of a heat insulating material, heat radiation from the resin at the part  11   a  adjacent to the fine configuration  14   a  is small. Therefore, the transfer accuracy of the fine configuration  14   a  is further improved. 
     Second Embodiment 
     See FIG.  2   
       FIG. 2  shows a mold  1 B according to a second embodiment of the present invention. According to the second embodiment, a ring-type heat insulator  15  is provided on the inner circumferential surface of the base  11  which is a wall of the cavity  30 , that is, between the surface processed layer  14  and the base  11 . 
     In the mold  1 B, the base  11  is made of a usual mold base material. The heat insulator  15  can be made of various materials with low coefficients of thermal conductivity, such as stainless steel, titanium alloy, nickel alloy, etc. Alternatively, ceramic, such as silicon nitride (Si3N4 with a coefficient of thermal conductivity of 20 W/m·K), alminium titanium (Al 2 O 3 —TiO 2  with a coefficient of thermal conductivity of 1.2 W/mK), etc., is usable for the heat insulator  15 . Also, heat resistant polymer, such as polyimide resin (with a coefficient of thermal conductivity of 0.28 W/mK), etc. is usable. Further, other materials can be used, and ceramic of various formulas can be used. The other parts of the mold  1 B are of the same structures and the same materials as those of the mold  1 A according to the first embodiment. 
     According to the second embodiment, since the heat insulating layer  13  exists between the core base  12  and the surface processed layer  14 , the temperature of resin injected into the cavity  30  can be kept well. Therefore, the transfer accuracy of especially the fine configuration  14   a  formed on the surface processed layer  14  is improved. 
     Further, since the heat insulator  15  exists between the surface processed layer  14  and the mold base  11  which is a wall of the cavity  30 , heat radiation from the resin at the part adjacent to the fine configuration  14   a  is small, and the transfer accuracy of the fine configuration  14   a  is further improved. 
     Third Embodiment 
     See FIG.  3   
       FIG. 3  shows a mold  1 C according to a third embodiment of the present invention. The mold  1 C comprises a heat insulator  16  instead of the heat insulator  15  provided for the mold  1 B according to the second embodiment. The materials usable for the heat insulator  15  can be also used for the heat insulator  16 . The other parts of the mold  1 C are of the same structures and of the same materials as those of the mold  1 B according to the second embodiment, and therefore, the effect of the third embodiment has the same effect as the second embodiment. 
     Fourth Embodiment 
     See FIG.  4   
       FIG. 4  shows a mold  1 D according to a fourth embodiment of the present invention. As  FIG. 4  shows, as well as the movable mold  10 , the fixed mold  20  comprises bases  21  and  22 , a heat insulating layer  23  and a surface processed layer  24 . The surface processed layer  24 , like the surface processed layer  14 , is finished in accordance with the configuration of an optical surface of a product (an optical element), and a fine configuration  24   a  is formed. 
     The heat insulating layer  23  and the surface processed layer  24  are made of the materials used for the heat insulating layer  13  and the surface processed layer  14 , which have been described in connection with the first embodiment. The bases  11  and  21  are made of a heat insulating material. The heat insulating material has been specifically described as the material of the base  11  in connection with the first embodiment. The core bases  12  and  22  are made of the material which has been described as the material of the base  12  in connection with the first embodiment. 
     According to the fourth embodiment, since the heat insulating layers  13  and  23  are provided respectively between the core base  12  and  14  and the surface processed layer  14  and between the core base  22  and the surface processed layer  24 , the temperature of resin injected into the cavity  30  can be kept well. Therefore, the transfer accuracy of especially the fine configurations  14   a  and  24   a  formed on the surface processed layers  14  and  24  is improved. 
     The bases  11  and  21  form walls of the cavity  30  and have areas  11   a  and  21   a , which are respectively adjacent to the surface processed layers  14  and  24 . Since the bases  11  and  21  are wholly made of a heat insulating material, heat radiation from the resin at the areas  11   a  and  21   a  respectively adjacent to the fine configurations  14   a  and  24   a  is small, and the transfer accuracy of the fine configurations  14   a  and  24   a  is further improved. Additionally, the resin is heat-insulated both on the upper and lower surfaces, and there is no fear that the molded product may have a bend. 
     Fifth Embodiment 
     See FIG.  5   
       FIG. 5  shows a mold  1 E according to a fifth embodiment of the present invention. The mold  1 E has ring-type heat insulators  17  and  27  on the inner circumferential surfaces of the bases  11  and  21  which are walls of the cavity  30 . The heat insulators  17  and  27  are made of the material which has been described as the material of the heat insulator  15  in connection with the second embodiment. 
     The bases  11  and  21  are made of a usual mold base material. The other parts of the mold  1 E are of the same structures and of the same materials as those of the mold  1 D according to the fourth embodiment. The fifth embodiment has the same effect as the fourth embodiment. 
     Sixth Embodiment 
     See FIG.  6   
       FIG. 6  shows a mold  1 F according to a sixth embodiment of the present invention. The mold  1 F is to mold a curved lens. The mold  1 F is composed of the same parts as the mold  1 C according to the third embodiment, and these parts are made of the same materials as those of the mold  1 C. Therefore, the sixth embodiment has the same effect as the third embodiment. 
     Molding Method 
     An injection molding method by use of one of the molds  1 A through  1 F is briefly described. 
     First, melted resin at a specified temperature (for example, amorphous polyolefine resin) is injected into the cavity  30 , and on completion of the injection, a pressure retention step starts immediately. The pressure retention step is a step of keeping a specified pressure applied to the resin so as to supply more resin to compensate shrinkage of the resin injected into the cavity  30  due to a fall in temperature. After the pressure retention step, a cooling (natural cooling) step starts. When at least the surface of the resin (molded product) cools down under a temperature to cause thermal deformation, the mold is opened, and the molded product is picked out of the mold by use of an eject pin or the like. 
     In the cavity  30 , immediately after completion of the resin injection, the temperature of the resin starts falling. In the molds  1 A through  1 E, however, the heat insulating layers  13  and  23  exist in the rear of the fine configurations  14   a  and  24   a , and the temperature of the resin injected into the cavity  30  can be kept. Also, the bases forming the cavity  30  are at least partly made of a heat insulating material, and heat radiation from the resin is inhibited. Therefore, the transfer accuracy of the fine configurations  14   a  and  24   a  is improved. 
     Other Embodiments 
     An injection mold and an optical element injection molding method according to the present invention are not limited to the above-described embodiment. 
     The details of the mold can be arbitrarily structured, and the materials named in the above embodiments are merely examples. In  FIGS. 1 through 6 , the mold  10  may be a fixed mold, and the mold  20  may be a movable mold. Alternatively, the mold may be a three-plate type further having an intermediate mold. 
     Although the present invention has been described in connection with the preferred embodiments above, it is to be noted that various changes and modifications are possible to those who are skilled in the art. Such changes and modifications are to be understood as being within the scope of the present invention.