this invention relates to blow molding apparatus for making plastic articles . it circulates cooling fluid through core rods for cooling blown articles from the inside , preferably with the blown article on the outside of a balloon that is permanently connected with the core rod . the connection of the balloon to the core rod is constructed so that there is no axial pull on the balloon during stripping of the blown article from the core rod and balloon . the invention provides constructions for admitting air between the balloon and a blown parison to break the vacuum when the balloon is to be collapsed . fluid is also circulated over the outside of the blown parison to cool it , and this can be done in a perforated shroud that serves as a blowing mold , particularly for flexible wall containers .