Insulator for spark plug, and method for manufacturing spark plug

A method for manufacturing an insulator for a spark plug which has a through hole extending in an axial direction for receiving a center electrode and a terminal electrode, and comprises a thick-walled portion having a wall thickness defined between the through hole and an outer peripheral surface greater than those of other portions in the axial direction, said method comprising:

• preparing a press pin for forming the through hole and a forming die having a cavity, the press pin comprising a rib-shaped pin-side spiral portion formed on an outer peripheral surface of a second position of the press pin;
• filling a raw powder into the cavity;
• arranging the press pin within the cavity by advancing a leading end of the press pin in the axial direction before, during, or after filling the powder;
• pressing the raw powder within the cavity along with the press pin, and obtaining a green body formed with a green body-side spiral portion to which a shape of the pin-side spiral portion is transferred;
• releasing the green body along with the press pin from the cavity;
• and
• retreating the press pin with respect to the green body while rotating the press pin relative to the green body around an axis, and extracting the press pin from the green body,
• wherein an outer diameter of the pin-side spiral portion is smaller than an outer diameter of an outer peripheral surface of a portion closer to a base end than the pin-side spiral portion;
• and
• wherein the second position is positioned such that the green body-side spiral portion is located within the thick-walled portion.