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.