Blow molding process

A process for the production of hollow bodies of thermoplastic material by blow molding comprising:

• forming a preform;
• initially expanding the preform within a blowing mold using an inert gas under high pressure thereby to cause the preform to bear completely against the inside surface of the blowing mold;
• reducing the internal pressure in the hollow body;
• treating the inside wall surface of the resulting hollow body within the blowing mold by introducing a reaction gas into the interior of the hollow body under a pressure which is substantially lower than the pressure used for intially expanding the preform, said reaction gas reacting with the plastic forming the inside wall surface of said hollow body thereby to render same at least substantially impervious;
• and after the necessary reaction time replacing the reaction gas by a flushing and cooling gas for substantially removing reaction gas constituents within the hollow body and for cooling thereof.