Abstract: A mold apparatus and method is used to form a solid injection molded plastic part having no internal voids, a Class A finish, a sink-free exterior appearance surface, and unevenly distributed structural detail extending from the opposite surface, the detail including free-standing reinforcement ribs, attachment bosses, and the like. In sequence, a seal groove in the mold cavity is reduced to a pressure below atmospheric (i.e., negative), a charge of hot thermoplastic is introduced into the mold cavity and seal groove whereby to form a gas seal ring, and pressurized gas is injected into the mold cavity. The pressurized gas forces the hot plastic from one mold half against the other mold half, and the gas seal formed by the plastic prevents gas on one side of the thermoplastic from reaching the appearance surface of the part.

Abstract: A gas assisted injection molding system and method in which quantity of an inert gas is applied both internally and externally to a thermoplastic material introduced into an injection mold cavity during formation of an injection molded part. The system includes first and second mold portions defining a mold cavity. An injection means injects a quantity of a thermoplastic material into the mold cavity to form an injection molded part within the cavity. At least one internal gas inlet is provided for introducing a first quantity of gas into a portion the thermoplastic material to form a gas channel therein. The gas within the gas channel presses the thermoplastic material against a mold parting line defined by the intersection of the mold portions and forms a seal between the mold cavity and the mold parting line.