Method of injection molding articles of thermoplastic materials

In a method of injection molding an article of thermoplastic material, the method including initially forcing molten thermoplastic material into a mold cavity of an injection mold in an amount sufficient for forming the article, the mold cavity having a surface, subsequently forcing a single flowable medium into the injection mold and thermoplastic material with a pressure which distributes the thermoplastic material uniformly over the surface of the mold cavity so as to form a hollow body, such that the thermoplastic material defines a surface, cooling the hollow body in the injection mold while maintaining the pressure of the medium therein, and releasing the pressure of the medium from the hollow body and removing the molded article from the injection mold, the improvement comprising conducting the flowable medium from a pressure reservoir into the mold cavity and to the surface of the thermoplastic material distributed over the surface of the mold cavity, preventing the flowable medium from flowing back out of the mold cavity and controlling the pressure of the medium existing within the injection mold and acting on the surface of the thermoplastic material distributed over the surface of the mold cavity at least temporarily until the thermoplastic material has cooled wherein the pressure of the medium is controlled from a lowest pressure level to a highest pressure level and then to a middle pressure level before the pressure of the medium is released from the hollow body.