PREPARATION OF MOLDED ITEM HAVING UNDERCUT

PURPOSE:To make it possible to mold a required shape on the surface, by closing a mold where an undercut is formed with a bag-like freely expandable and contractable member, acting a fluid pressure in the expandable and contractable member, removing the fluid pressure after molding, contracting and peeling off the expandable and contractable member, and thereafter opening the mold. CONSTITUTION:A cavity 3 and a core are opened and a pressurized fluid is sent in an expandable and contractable member 5 from a tube 8. The pressure is made higher than the pressure of the resin when being molded. In this condition, according to the usual blow molding operation, a parison is placed between the cavity 3 and a core 4, which are then closed. The parison is expanded by sending a compressed air and is contacted under pressure with the inner face of the mold. At this time, the expandable and contractable member 5 acts as a part of the core of the mold and an undercut part 3 is formed on a molded item 1 in accordance with the shape of the expandable and contractable member 5. Then, the pressurized fluid in the expandable and contractable member 5 is sucked by means of the tube 8. The expandable and contractable member 5 is thereby peeled off from the undercut part 2 of the molded item 1 and the cavity 3 and the core 4 are opened to take out the molded item 1.