Patent Publication Number: US-2009236016-A1

Title: Method for manufacturing glass molding die

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation Application of PCT Application No. PCT/JP2007/073955, filed Dec. 12, 2007, which was published under PCT Article 21(2) in Japanese. 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2006-337367, filed Dec. 14, 2006, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for manufacturing a glass molding die requiring precision machining, and specifically to a manufacturing method under which a die maintains its shape with high precision. 
     2. Description of the Related Art 
     In the field of plastic molding, there are established techniques for accuracy machining of molding dies. Using such techniques, optical devices having fine shapes, such as diffraction gratings, are mass-produced. These dies are manufactured by coating the surfaces of a stainless steel substrate with electroless Ni—P plating, followed by precise machining of the plated layer with a diamond bite. 
     BRIEF SUMMARY OF THE INVENTION 
     However, if these dies are used for glass molding, the Ni—P layer formed by electroless plating may be cracked. The phenomenon is ascribable to the molding temperature. More specifically, the N—P layer is in an amorphous state after plating, and is crystallized when heated to about 270° C. or higher. At this point, the plated layer causes volume shrinkage, and is cracked by tensile stress. 
     In order to solve the problem, for example, in Jpn. Pat. Appln. KOKAI Publication No. 11-157852, a substrate having a coefficient of thermal expansion of 10×10 −6  to 16×10 −6  K −1  is subjected to plating, followed by heat treatment at 400 to 500° C. However, although the substrate has a coefficient of thermal expansion equal to that of the Ni—P plated layer, the volume shrinkage accompanied by the crystallization occurs only in the plated layer during heat treatment, which may result in cracking in the plated layer due to great tensile stress. 
     In addition, if the die is heated to a high temperature during use, the die causes plastic deformation, which results in the failure to maintain the shape of the die with high precision. 
     The present invention is intended to provide a method for manufacturing a glass molding die. According to the method, cracking in the surface coating layer at the molding temperature and plastic deformation of the die are prevented, whereby the die maintains its shape with high precision, and has a longer life. 
     In order to solve the problems and accomplish the object described above, the method for manufacturing a glass molding die according to the present invention has the following aspects. 
     Forming a substrate by hardening a steel material containing 0.3 wt % or more and 2.7 wt % or less of carbon and 13 wt % or less of chromium, and further containing at least one additive selected from 0.5 wt % or more and 3 wt % or less of molybdenum, 0.1 wt % or more and 5 wt % or less of vanadium, and 1 wt % or more and 7 wt % or less of tungsten, and then tempering the steel material at a temperature of 400° C. or higher and 650° C. or lower; forming a surface coating layer composed of an amorphous Ni—P alloy on the surface of the substrate; and heating the surface coating layer thereby rendering the surface coating layer an eutectic structure composed of Ni and Ni 3 P. 
     Forming a substrate by hardening a steel material containing 0.3 wt % or more and 2.7 wt % or less of carbon and 13 wt % or less of chromium, and further containing at least one additive selected from 0.5 wt % or more and 3 wt % or less of molybdenum, 0.1 wt % or more and 5 wt % or less of vanadium, and 1 wt % or more and 7 wt % or less of tungsten, and then subjecting the steel material to subzero treatment; forming a surface coating layer composed of an amorphous Ni—P alloy on the surface of the substrate; and heating the surface coating layer thereby rendering the surface coating layer an eutectic structure composed of Ni and Ni 3 P. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a block diagram schematically showing a method for manufacturing a glass molding die according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a block diagram schematically showing a method for manufacturing a glass molding die according to one embodiment of the present invention. The glass molding die is manufactured by the following process. 
     The substrate used herein is a steel material containing 0.3 wt % or more and 2.7 wt % or less of carbon and 13 wt % or less of chromium, and further containing at least one additive selected from 0.5 wt % or more and 3 wt % or less of molybdenum, 0.1 wt % or more and 5 wt % or less of vanadium, and 1 wt % or more and 7 wt % or less of tungsten. 
     The substrate is subjected to rough machining (ST 1 ), and then to hardening and high temperature tempering (ST 2 ). Subsequently, the substrate is pretreated before plating (ST 3 ), and then subjected to electroless plating to form a surface coating layer (plated layer) composed of an Ni—P alloy (ST 4 ). Subsequently, the substrate and the surface coating layer are subjected to heat treatment (ST 5 ) thereby crystallizing the surface coating layer, and tempering the substrate. Thereafter, the substrate and the surface coating are finished (ST  6  and ST 7 , respectively), and then the surface coating layer is coated with a releasing film (ST 8 ). 
     The substrate used in the manufacturing method according to an embodiment of the present invention is a steel material containing Mo, V, or W for improving the high-temperature hardness. Therefore, the surface coating layer will not be cracked during high-temperature tempering. The reason for this is that the substrate contains a large amount of residual austenite immediately after hardening, but the residual austenite is transformed into low carbon martensite and martensite by high-temperature tempering. 
     The temperature of the high-temperature tempering is 400 to 650° C. or less. If the temperature is lower than 400° C., the residual austenite is not so effectively reduced, and if higher than 650° C., the substrate is markedly softened. The high-temperature tempering may be replaced with subzero treatment. Subzero treatment is also effective for transforming residual austenite into martensite. 
     The surface coating layer is formed with an Ni—P alloy such as Ni—P, Ni—P—B, or Ni—P—W. These structures are amorphous or partially amorphous after plating, and is transformed into a completely crystallized mixed structure composed of Ni and Ni 3 P after heating at a temperature of about 270° C. or higher. 
     The temperature of the heat treatment is greater than or equal to the working temperature of the die (more specifically, the glass molding temperature). If the heat treatment temperature is lower than the working temperature of the die, the dimension of the die can vary during use, which results in the deterioration of the dimensional accuracy of the molded product. If the heat treatment temperature is too high, the plated surface is affected. Therefore, the upper limit of the heat treatment temperature is about 700° C. 
     The reason for the use of the steel material having the above-described composition as the substrate is as follows. The C content is 0.3 wt % or more and 2.7 wt % or less. If the C content is less than 0.3 wt %, the volume shrinkage of the substrate during tempering is insufficient. On the other hand, if the C content is more than 2.7 wt %, the volume shrinkage of the substrate is sufficient, but the toughness of the substrate deteriorates. 
     The Cr content is 13 wt % or less. If the Cr content is more than 13 wt %, the residual austenite is poorly decomposed. The lower limit of the Cr content is not particularly limited. 
     The contents of Mo, V, and W as additives are 0.5 wt % or more and 3 wt % or less, 0.1 wt % or more and 5 wt % or less, and 1 wt % or more and 7 wt % or less, respectively. If the amount of these additives is too small, the substrate has insufficient high-temperature hardness, and may cause plastic deformation under pressure. Excessive addition of the additives results in an increase in cost, so that the upper limits are defined. 
     Substrates with different compositions were subjected to electroless Ni—P plating to form dies coated with a 100-μm-thick coating. These dies were subjected to heat treatment and molding, and the incidence of cracking during molding, and whether the substrates caused plastic deformation during glass molding were recorded. Table 1 lists the composition of the substrates, tempering temperature, the incidence of cracking, and the assessment of whether the substrates caused plastic deformation. The specimen 7 as a comparative example is a plastic molding die subjected to conventional heat treatment. The molding temperature was 550° C. for all the specimens. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Composition of substrates, tempering temperature, 
               
               
                 incidence of cracking, and plastic deformation 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Mo, V, W 
                   
                 Incidence 
                 Plastic 
               
               
                 Substrate 
                 C content 
                 Cr content 
                 content 
                 Tempering 
                 of cracking 
                 deformation 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Specimen 1 
                 1.2 
                 6.0 
                 None 
                 None 
                 0/5 
                 Deformed 
               
               
                 Specimen 2 
                 1.2 
                 6.0 
                 Mo: 1.0, V: 1.0 
                 None 
                 3/6 
                 None 
               
               
                 Specimen 3 
                 1.2 
                 6.0 
                 Mo: 1.0, V: 1.0 
                 None 
                 1/5 
                 None 
               
               
                 Specimen 4 
                 1.2 
                 6.0 
                 None 
                 580° C. 
                 5/5 
                 Deformed 
               
               
                 Specimen 5 
                 1.2 
                 6.0 
                 Mo: 1.0, V: 1.0 
                 580° C. 
                 0/5 
                 None 
               
               
                 Specimen 6 
                 1.2 
                 6.0 
                 Mo: 1.0, V: 1.0 
                 580° C. 
                 0/5 
                 None 
               
               
                 Specimen 7 
                 0.3 
                 14.0 
                 V: 0.3 
                 580° C. 
                 5/5 
                 None 
               
               
                   
               
            
           
         
       
     
     As is evident from the results in Table 1, the dies manufactured according to the method of the present invention (specimens 5 and 6) caused neither cracking nor plastic deformation. 
     As described above, through the use of the method for producing a glass molding die according to an embodiment of the present invention and a glass molding die manufactured by the method, cracking in the surface coating layer at the molding temperature and plastic deformation of the die are prevented, and the die maintains its shape with high precision and has a longer life. 
     The present invention is not limited to the above-described embodiment. For example, the substrate and surface coating layer may be subjected to heat treatment after finishing the substrate and surface coating layer. In addition, various modifications may be made without departing from the scope of the present invention. 
     According to the present invention, cracking in the surface coating layer of a die at the molding temperature and plastic deformation of the die are prevented, whereby the die maintains its shape with high precision, and has a longer life.