Method for the injection molding of internally hollow parts

Method for the injection molding of internally hollow parts (P) by means of an apparatus comprising a mold consisting of a matrix (1) and a cooperating punch (2), both of which delimiting an inner space (3) communicating, on one side, with a nozzle (4) for the injection of melt (F) of thermoplastic material and with a pressurized gas supplying pipe (5), controlled by a first valve (6) for supplying nitrogen (N), and on the other side with a connecting duct (7) that places said inner space (3) in communication with a secondary compartment (8), a second valve (9) acting on the connecting duct (7) for the closure and opening thereof and a perforating element (10) that perforates a part (P) in the connecting duct (7), the method comprising the steps of:

• injecting melt (F) of thermoplastic material in said inner space (3) between the punch (2) and matrix (1),
• opening the second valve (9) in order to place said inner space (3) in communication with the secondary compartment (8),
• stopping the injection of the melt (F) and opening the first valve (6) for supplying nitrogen (N) in said inner space (3) in order to create a cavity (13), until the passage of the nitrogen (N) in the secondary compartment (8),
• stopping the supply of nitrogen (N) and simultaneously perforating the part (P) being molded in the section of the connecting duct (7) by means of the perforating element (10) for the outflow of nitrogen (N) from the mold and the subsequent possible collection thereof.
• characterized in that it also comprises a supply (A) of pressurized air into said gas pipe (5) for washing towards the atmosphere after stopping the supply of nitrogen (N) and the simultaneous perforation of the part (P) being molded in the section of the connecting duct (7).