in producing hollow bodies by blow molding from thermoplastic material , a preferably tubular preform is firstly formed , and is then expanded within the blowing mold under the effect of an inert gas under high pressure , until it bears entirely against the inside surface of the blowing mold . thereafter , a reaction gas such as fluorine is introduced into the hollow body under a considerably lower pressure , while the hollow body is still in the mold , to treat the internal surface of the hollow body to render it impervious to gas and / or liquid . after the reaction has elapsed , a flushing and cooling gas is introduced into the hollow body and is then removed from the blowing mold after the hollow body has sufficiently set .