Spark plug insulator production method, insulator, molding die

A spark plug insulator production method for manufacturing a tubular insulator for a spark plug having a large-diameter portion having the largest outside diameter, a portion smaller in outside diameter than the large-diameter portion, and an axial hole extending in a direction of an axial line, the method comprising:

• disposing a rod member in a space of a rear mold, the rear mold having an internal surface whose shape is identical to an external shape of a rear molded portion located rearward, in the direction of the axial line, of a molded portion corresponding to the large-diameter portion of the insulator, wherein the internal surface defines the space of the rear mold, the rear mold comprising a plurality of mold pieces which form different portions of the internal surface, the rod member having an external surface identical in shape to a portion of the axial hole of the insulator corresponding to the rear molded portion;
• injecting a material into a space formed between the internal surface of the rear mold and the external surface of the rod member, thereby molding the rear molded portion;
• and
• releasing the rear molded portion from the rear mold by disassembling the rear mold into the plurality of mold pieces, wherein
• the plurality of mold pieces include a first mold piece which forms a first partial internal surface of the internal surface of the rear mold along the entire circumference of the internal surface, and a second mold piece which is located forward, in the direction of the axial line, of the first mold piece and forms a second partial internal surface of the internal surface of the rear mold along the entire circumference of the internal surface, and
• the step of releasing the rear molded portion from the rear mold includes starting to move the first mold piece rearward in the direction of the axial line in relation to the rear molded portion, and, subsequently starting to move the first mold piece, starting to move the second mold piece rearward in relation to the rear molded portion, thereby disassembling the rear mold into the plurality of mold pieces.