Metal mold
2008-08-20

Metal injection molding or (MIM) is a manufacturing process which combines the versatility of plastic injection molding with the strength and integrity of machined, pressed or otherwise manufactured small, complex, metal parts. Competing processes include pressed powder sintering, investment casting, turning and machining. The process involves combining fine metal powders with a plastic binders which allow the metal to be injected into a mold using standard plastic injection molding machines. After the part is molded and before the binders are removed, the part is referred to as a 'green part'. The next step is to remove the binders with solvents and thermal processes. The resultant metal part is sintered at temperatures great enough to bind the particles but not melt the metal. The products of metal injection molding are up to 98% as dense as wrought iron and used in a broad range of applications (including medical, dental, firearms, aerospace, and automotive just to name a few.) The window of economic advantage in metal injection molded parts lies in the complexity and small size of the part. Tolerances as small as +/-.003" per linear inch can usually be held without secondary processes. The difficulty of fabrication through other means may make it cost inefficient or even impossible to manufacture otherwise. Increasing complexity for traditional manufacturing methods typically does not increase cost in a metal injection molding operation due to the wide range of features possible through injection molding (threads (both internal and external), miniaturization, branding). Metal mold for forming TECHNICAL FIELD This invention relates to a metal mold for forming, a method for the preparation of the metal mold for forming and a molded article obtained on forming by the metal mold for forming. More particularly, it relates to a metallic forming mold for forming, having a transfer surface for applying decorative processing to a molded article on transcription to the surface of the molded article, a method for the preparation of the metal mold for forming, and a molded article prepared using this metallic forming mold for forming. BACKGROUND ART Among the molded products, prepared on injecting a molding material, such as molten resin, charged into a cavity of a forming metal mold, and subsequently on allowing the so charged molten resin to be cooled and solidified, there are those the surfaces of which are decoratively worked for affording the feeling of the high grade product so that the finished articles appear as if they are formed of a metallic material. Examples of the decorative machining include mirror surface finishing, which works the product surface to a smooth and lustrous finish, a so-called spin mark machining, which forms a large number of fine parallel concentric lines on the product surface, and a so-called hairline machining, which forms a large number of fine parallel straight lines on the product surface. The molded products with decorative surface machining are mostly formed using a metal mold provided with a transfer surface prepared by electroplating. The conventional manufacturing method for a forming metal mold, used for producing a molded article with such decorative machining, is shown in FIGS. 1 to 4. In a conventional manufacturing method, a master 101 is first prepared, as shown in FIG. 1. This master 1 is formed of a metal material, such as brass or beryllium copper, and has a diamond cut surface 102 formed on a rim portion of its upper surface by diamond cutting. The upper surface of the master 101 is formed with a plural number of fine concentric scores for forming spin marks on the molded product to provide a spin forming surface 103. A plating layer 104 then is formed by electroplating on the entire surface of the master 101, as shown in FIG. 2. The plating layer 104 is formed of, for example, chromium, to a thickness of 5 to 6 mm. The master 101 and the plating layer 104 then are separated from each other to form a recess for filling 105 on the plating layer 104, as shown in FIG. 3. By separating the master 101 and the plating layer 104 from each other, the diamond cut surface 102 and the spin forming surface 103 are transcribed on corresponding surfaces of the plating layer defining the recess for filling 105. The surface to which the diamond cut surface 102 has been transcribed is formed as a mirror surface processing surface 106 for transcription to the surface of a molded article for providing mirror surface finishing to the molded article, whilst the surface to which the spin forming surface 103 has been transcribed is formed as a spin processing surface 107 for transcription to the surface of the molded product to produce spin marks thereon. The plating layer 104 is inserted into and fixed in an insertion recess 109 of a base mold of the forming mold 108, as shown in FIG. 4. The base mold 108, with the plating layer 104 secured thereto, is set and assembled with a fixed forming mold, not shown, to produce a metallic forming mold. A molding material is charged into the recess for filling 105. When the molding material is cooled and cured, the metal mold is opened to take out the molded product from the recess for filling 105. On the molded article, thus prepared, there are transcribed the mirror surface processing surface 106 and the spin processing surface 107, such that the molded article appears as if it is molded of a metal material. With the conventional method, described above, the master 101 is prepared of a metal material, such as brass or beryllium copper, and the plating layer 104 is formed on its surface by electroplating, in order to apply decorative processing to the surface of a molded article. Usually, the plating layer 104 can be formed to a preset thickness by electroplating only after a processing time interval as long as approximately two months, thus raising an inconvenience of considerably protracted manufacturing time. Apart from the problem of the protracted manufacturing time, the plating layer 104 needs to be formed in addition to the master 101, thus raising another inconvenience of increased production cost. DISCLOSURE OF THE INVENTION It is therefore an object of the present invention to provide a metallic forming mold, a method for the preparation of the metallic forming mold and a molded article obtained on forming by the metallic forming mold, which are novel and which are free from the above-mentioned inconveniences of the prior art techniques It is another object of the present invention to provide a metallic forming mold, method for the preparation of the metallic forming mold and a molded article obtained on forming by the metallic forming mold, with which it is possible to shorten the manufacturing time length of the metallic forming mold and the molded article and to reduce the manufacturing cost. In one aspect, the present invention provides a forming metal mold including a base mold, a first forming mold having a molding recess, a second forming mold secured to the base mold along with the first forming mold and having a charged opening communicating with the molding recess, a transfer surface section formed on an opening edge surface of the molding recess of the first forming mold for transcribing decoration onto a surface of a molded article, and a charged opening formed in the second forming mold. The charged opening forms a cavity, in which a molding material is to be introduced, along with the molding recess. In another aspect, the present invention provides a forming metal mold including a base mold, a first forming mold having a molding recess, a second forming mold secured to the base mold along with the first forming mold and having a charged opening communicating with the molding recess, a first transfer surface section formed on one end of the molding recess of the first forming mold for transcribing decoration onto a surface of a molded article, and a third forming mold arranged between the base mold and the first forming mold and inserted from the opposite end of the molding recess. The third forming mold is secured to the base mold along with the first and second forming molds and having a second transfer surface section in continuation to the transfer surface section. A transfer surface for applying decorative processing on the molded article on transcription to its surface is formed within the molding recess in continuation to the opening edge of the molding recess. The first and second forming molds are combined together so that the molding recess continues to the charged opening of the second forming mold and the first and second formed molds thus combined together are secured to the base mold. In another aspect, the present invention provides a method for manufacturing a forming metal mold including forming a first transfer surface section on one end of the molding recess of a first forming mold for transcribing the decoration to the surface of a molded article, introducing a second forming mold from the opposite end of the molding recess of the first forming mold, securing the first and second forming molds such that the first and second forming molds are clamped in position between a third forming mold having a charged opening and a base mold. In another aspect, the present invention provides a method for manufacturing a forming metal mold including forming a first transfer surface section on one end of the molding recess of a first forming mold for transcribing the decoration to the surface of a molded article, introducing a second forming mold from the opposite end of the molding recess of the first forming mold, securing the first and second forming molds such that the first and second forming molds are clamped in position between a third forming mold having a charged opening and a base mold so that a second transfer surface section formed in the second forming mold continues to the first transfer surface section and so that the charged opening continues to the first transfer surface section. The charged opening and the first and second transfer surface sections together delimit a cavity. The forming metal mold and the manufacturing method therefor according to the present invention are not in need of a process of forming a plating layer on the master surface. In another aspect, the present invention provides a molded article obtained on forming by a forming metal mold on cooling and solidifying a molding material charged into a cavity defined by a molding recess of a first forming mold having a transfer surface and by a charged opening of the second forming mold. The molded article includes a transfer section obtained on transcription of the transfer surface of the first forming mold. In another aspect, the present invention provides a molded article obtained on forming by a forming metal mold on cooling and solidifying a molding material charged into a cavity defined by a charging portion of a molding recess of a first forming mold having a transfer surface and by a charged opening of the second forming mold. The molded article includes a transfer section obtained on transcription of the transfer surface of the first forming mold in an insertion positioning section of which is introduced and placed a third forming mold. The molded article, obtained using a forming metal mold according to the present invention, may be prepared with high transfer performance using a forming metal mold prepared without requiring electroplating.