Method of making seamless hollow molded articles

The method of forming a hollow article from moldable settable material by molding the material about a core in a segmental pressure mold having complementary parting surfaces, respectively, and comprising:

• charging with unset settable moldable material a segmental mold having a cavity shaped to impart a predetermined exterior configuration to the article and in which cavity a removable core having a shape to impart a predetermined interior configuration to the article is supported, said core having a portion disposed between at least a portion of said parting surfaces and shaped to impart to at least part of the article at its periphery, aligned spaced mateable edges, respectively, from between which the core can be withdrawn from the article when mold pressure is relieved and the mold is opened sufficiently, and at least one of which edges, at each aligned unit of the length of said edges, is disposed beyond the adjacent complementary parting surfaces of the mold segments themselves, after removal of the core, a distance sufficient so that after removal of the core, the spaced mateable edges will be juxtaposed in face to face relation under pressure by movement of the mold segments to fully closed positions;
• confining and molding the material by the mold segments and core under predetermined molding pressure in the space between the walls of the cavity and core;
• subjecting the confined material, except for a small edge forming portion, which edge portion begins at the mateable faces of said edges and continues therefrom for a slight depth, to a predetermined temperature required for effecting the setting reaction of the material, continuously until the reaction has proceeded to a high degree which is beyond the scorch stage of the material and is sufficient to create a skin over the molding surfaces and impart structural and dimensional stability thereto;
• concurrently maintaining said edge forming portion of the material at a lesser temperature so as to initiate the setting reaction thereof and carry said setting reaction to a lesser degree, which lesser degree is below the skin forming stage so that the mateable edges are free from skin effect, are firm enough to transmit substantial pressure upon closure of the mold, and are fusible and bondable;
• then opening the mold and immediately removing the core therefrom, and immediately reclosing the mold under molding pressure;
• immediately after reclosure of the mold, subjecting all of said material, including said edge forming portion, to said predetermined temperature;
• continuing the application of heat until the setting reaction of all of the material is carried to completion;
• and
• then opening the mold and removing the molded material from the mold.