Device for injection moulding tubular preforms made of a thermoplastic

In a device for injection molding a tubular perform of a thermoplastic, the perform having a longitudinal axis, a small wall thickness and a great length parallel to the longitudinal axis, which device is composed or an injection-molding press equipped with an injection nozzle, and at least one injection mold, the injection mold including an external die and an internal core, the injection nozzle extending into the die, the internal core being insertable into the die in the direction of the longitudinal axis for causing the die and the core to define a cavity in which the preform is formed, the press including means to fill the cavity with injected thermoplastic from the nozzle, the die being composed of two die-halves which are movable relative to one another transverse to the longitudinal axis, the improvement wherein when said core is moveable into said die to define said cavity, said core and said die are supported by supporting means to provide for progressive movement of said core and said die relative to one another parallel to the longitudinal axis to expand said cavity axially by said progressive movement during an injection by said nozzle of a preform from a staring position in which the length of the cavity is less than the length of the tubular preform to a final position in which the cavity corresponds in length, shape and volume to the tubular preform, during filling of the cavity with injected thermoplastic via said nozzle, with the ratio of the volume of the cavity in the starting position to the volume of the cavity in the final position being 1:1.5 to 1:100.